Well, if you are referring to the third clause, the majority opinion is that theos is neither definite nor indefinite but qualitative (which disfavors both "God" and "a god" in translation). This is the difference between saying I am woman (as in the Helen Reddy song) and I am a woman. Notice that the former is paralleled with other qualitatives in the Reddy song: "I am strong / I am invincible / I am woman". This is a very different statement than simply saying that "I am a member of the class customarily called 'woman' ". It emphasizes the nature and experience of womanhood in a way counter to patriarchal stereotype. This is analogous to how theos is used in John 1:1. Just as Helen declares "I am woman", the author of the Fourth Gospel affirms that the Logos is theos, i.e. the nature of the Logos is that of God. The NWT translation simply makes the Logos a member of a class of god, which fails to capture the qualitative force of the syntax. In the verse theos is 1) non-articular, i.e. anarthrous, 2) it is nominative, 3) it takes a copular predicate, and 4) it is in a preverbal predicate position. In Acts 28:6, where theos is clearly indefinite and non-qualitative ("they said that he was a god"), non-articular theos is NOT nominative and it FOLLOWS the verb rather than precedes it. The closest parallel to the verse is found in 1 John 4:8 (also Johannine in style) which says ho theos apagé estin, "God is love". As in John 1:1c, the qualitative noun (here apagé) is 1) non-articular (usual in the case of this noun, as it is abstract), 2) nominative, 3) it takes a copular predicate, and 4) it is in a preverbal predicate position. The qualitative force of the predicate noun highlights love as defining the nature of God. Similarly, theos defines the nature of the Logos in John 1:1. There is another parallel to both John 1:1 and 1 John 4:8 in the epistles of Ignatius of Antioch (writing in the early second century AD):
"None of these things escapes your notice, if you have perfect faith and love toward Jesus Christ. For these are the beginning and end of life (zóés): faith is the beginning (arkhé men pistos), and love is the end (telos de agapé), and [the two existing in unity] are God ([ta de duo en enotéti genomena] theos estin)" (Ephesians 14:1).
The thought is the same as in 1 John 4:8 although with the predicates reversed ("love is God" rather than "God is love"). But with the predicates flipped, we here have an exact equivalent to the theos in John 1:1c: 1) theos is non-articular, 2) it is nominative, 3) it takes a copular predicate, and 4) it is in a preverbal predicate position. As in John 1:1, God is the nature borne by the subject (the Logos in John 1:1 and faith and love in unity in Ignatius). There is also a parallel here to the sharing of nature between two entities in unity; in John 1:1 the nature of theos is shared between God (ho theos) and the Logos (ho logos), and in Ignatius the nature of theos is shared between faith and love.
So how can this thought be best expressed in English? Notice that the translation of Ignatius above (that of Michael Holmes) renders the theos as "God", as most English translations do in John 1:1. The problem with this translation is that in English, "God" is uniquely the name of a specific unique entity. This overrides the qualitative nuance in a way that saying "I am woman" does not (and it facilitates the misunderstanding that the Logos is simultaneously the God he is with). And since the noun theos is specifically amenable to theological interpretation, its translation is inherently problematic. Some have tried to translate the theos via the adjective "divine". This gives full credit to the qualitative nuance. But this is problematic as well. The qualitative force in adjectives is usually weaker than that of qualitative nouns (which are more marked). So in the Helen Reddy song, saying "I am womanly" or "I am female" fails to express the much stronger import that "I am woman" expresses. It is the same with 1 John 4:8. "God is love" is more powerful than saying "God is loving" or "God is lovelike". The evangelist did not use the adjective theios "divine", he used a stronger qualitative noun. "The Word was divine" is weaker because it admits the reading that the Logos is less divine than God (ho theos). The idea instead is that the Logos and God are equally divine. The idea in 1 John 4:8 is that "Everything love is, God is". Similarly, the idea in John 1:1 is "Everything theos is, the Logos is". This accords well with the theological idea repeated throughout the gospel that although the Son is subject to the Father in their own mutual relationship, he is equal to the Father in nature and activity (see 5:18 which says "Whatever the Father does, the Son does too" and the preceding verse in which the evangelist — not Jesus' opposers — asserts that the Son was indeed "making himself equal to God"), thus Jesus declares "Anyone who has seen me has seen the Father .... I am in the Father and the Father is in me" (14:9-10).
So it is hard to express this thought without a paraphrase. The NEB renders as "what God was, the Word was"; Philip Harner suggests "the Word had the same nature as God". The usual translation "The Word was God" is okay as long as it is understood in the same sense as Helen Reddy saying "I am woman", but it is hard for most English readers to sustain that qualitative understanding without succumbing to a definite reading that takes "God" to be equivalent to the Father whom the Logos was with. That is due to fact that "God" is almost exclusively used as the name of a specific deity in English, as opposed to theos in Greek which had a more flexible usage. As a third example of qualitative theos emphasizing nature, consider the following from Melito of Sardis (middle of the second century AD), who was clearly influenced by the Fourth Gospel:
"For indeed the Law has become reason (ho nomos logos egeneto), and the old new (ho palaios kainos [egeneto]), and the commandment grace (hé entolé kharis [egeneto]), and the impression truth (ho tupos alétheia [egeneto]), ... and the sheep man (to probaton anthrópos [egeneto]), and the man God (ho anthrópos theos [egeneto]). ... He rose from the dead as God (anesté ek nekrón hós theos), being by nature God and Man (phusei theos ón kai anthrópos)" (Melito of Sardis, Peri Pascha 7-8).
Notice that phusis "nature" is even used in the same clause with the qualitative nominative predicate theos.