It's a big help if you have a baseline ability to read Java, as it's likely the example code for libraries you'll want to use is in Java. You don't need to be able to create a baroque class hierarchy yourself as long as you can navigate one well enough to figure out the library calls you need to use.

Clojure is a fairly gentile introduction to functional programming, I think. You can do imperative-style things with the reference types, so you don't need to completely learn how to structure the logic of a program.