This is often a question asked by non-lisp devs. It's not Clojure specific - you just use the code itself.

Open a REPL, and start working. You can just execute the code in realtime as you're refactoring. The size of the codebase is largely irrelevant.

Ironically, in Clojure the compiler _does_ tell you what is working and what isn't when you work this way :)

And anyway, you write tests don't you?