GNU Emacs is an extensible, customizable text editor—and more. http://www.gnu.org/software/emacs/
- Homebrew
brew install emacs --cocoa --srgb
You might want to copy the app into the applications directory, otherwise Spotlight and other similar tools won't be able to see it:
cp -r /usr/local/Cellar/emacs/24.3/Emacs.app /Applications/
The clojure-mode, clojure-test-mode, nrepl and paredit-mode are essential for a fluid Clojure development. But, from there it's up to you, the options are endless.
You'll have problems with accents given the option key has a special meaning in Emacs. The solution is here.