Well, hmm, I don't think that's quite right either. There is a category where the objects are clojure types and the arrows are clojure functions, but I don't see how the threading operator embodies it. There could totally be some perspective I'm missing, though.
No, I think I'm just playing more fast and loose than I should. I don't think it fits into any particular semantic mould because it's really a syntactic thing—it is a list processing function that's applied to source code represented as lists. The basic usage pretty much traces out a path in the Clj category, so perhaps if you built Paths(Clj) then (->>) is a forgetful functor from Paths(Clj) -> Clj. In which case it's almost a monad, since if you play with Paths and (->>) you can turn them into a forgetful/free adjunction pair and make a monad.
