That most certainly is auto-fill-mode at work. To change the column at which emacs will split the line you can use M-x set-fill-column. If you want to change it permanently you can do it in your clojure-mode hook, something like:
(add-hook 'clojure-mode-hook (lambda () (set-fill-column 120)))
On the other hand, if you want to turn it off something like:
(add-hook 'clojure-mode-hook (lambda () (auto-fill-mode 0)))
Should do the trick.