It's simply a matter of semantics. The WT can define "clergy" in a self-serving manner by suggesting that they are different than other "false religions." For instance, they can claim that clergy are salaried professionals and because the WT only employs volunteers, they don't have a clergy class (the whole issue of whether stipends and green handshakes count as salary aside). Or, they can claim that "clergy" are people who perform religious rituals--such as mass. Because JWs don't engage in mass-like religious rituals, they don't have a clergy class.
That aside, it can be argued that JWs are even more stratified and hierarchical than the Catholic Church, itself. The social system of rewards and promotions in Watchtowerland is one gigantic pyramid.
Publishers--->Auxiliary Pioneers--->Regular Pioneers--->Special Pioneers--->Ministerial Servants--->Elders---->Bethelites--->CO's--->DOs--->Bethel Heavies--->Governing Body
JWs have a clearly defined, hierarchical chain of command. At the top, the governing body acts a Pope-by-committee. It's true that they don't necessarily claim to be infallible, but their orders carry the weight of gospel and violations of them are punishable by excommunication. So, again, it's all about semantics. The chain-of-command is more rigid within the JWs than it is in the Catholic Church, which they so love to bash and that should be the bottom line, not whether they have a "clergy class" as defined by the WT itself.