By contrast, there's a level of indirection between Clojure and the underlying JVM. And so, for much the same reasons as it's a good idea to know some C when doing systems programming, you won't really understand what Clojure is doing (especially not the stack-traces!) unless you understand Java.
There's also the practical point that, if you're working on the JVM, you'll be wanting to make use of the massive class libraries available for it. They're all written for native Java, so you'll have to be familiar with Java semantics to use them effectively. (Although the Lispy macro goodness means you can then wrap them up inside a concise functional abstraction when you're using them).
If you still want to look at the JVM (for fun, or for implementation reasons), I'd say a good knowledge of Java is enough to just jump head-first into the specification: http://java.sun.com/docs/books/jvms/second_edition/html/VMSp...
Don't get me wrong, I love the language to bits, but a lot of its beauty is in how it takes the existing Java model and does something cool with it - and to understand that, you need to understand what it's working with.