There should be no major problem if someone knows both Java and a Lisp. Either reading Programming Clojure or Pratical Clojure should be enough to start with, or maybe this tutorial http://java.ociweb.com/mark/clojure/article.html (I didn't read it but it supposed to be good)
For somebody who don't know neither Java neither a Lisp, I would suggest to understand the following concepts in Java and the JVM before doing the suggestions in the second point (though PCL could be read before learning Java to see why Lisps are so awewome):
- collections (java.util.List, hashmap, set etc.) since each Clojure collection implements a Java interface. This is useful for interoperability.
- interfaces, inheritance and access specifiers (because then you can understand why you can not override a Java protected method from a Clojure proxy)
- some of the most common API like regular expressions, syntax for printf etc.
