At least some of the recent Clojure implementations are based on ClojureScript's compiler, which from what I understand is relatively moduler: http://blog.fogus.me/2012/03/27/compiling-clojure-to-javascr...
Lisps in general are appealing to implement. Clojure's core language is about as small as Scheme's, and there are a lot of Scheme implementations too. The parts of Clojure that set it apart from other Lisps (STM, persistent data structures) aren't always ported. I don't think any of the recent approaches to targeting C (Clj->Scheme->C, Clj->C) include STM or persistent data structures. I'd be happy to be pleasantly surprised though.