Mergesort implementation in Clojure
A few points, though I'm not a Clojure expert:


Indentation. You use four-space indentation, but two-space indentation is more common for Lisp in general and also generally preferred for Clojure. (there seems to be a Clojure community style guide)
Instead of take-last, you may want to use drop. Since you're traversing the whole list anyway, you could also just use split-at (though that's basically the same as using both take and drop).