Accessing a function bound to a var in clojure

So if I understand this correctly, when I write:

(defn foo [x] (+ (* (- x 3) 2) (- x 3)))

foo gets bound to:

(fn [x] (+ (* (- x 3) 2) (- x 3)))

How do I access the function from foo? My intention is to change something in the function and return a new function.