dynafoo.clj

(defn foo [] 42)

(foo)

(defn mk-foo []
  foo)

(defn mk-bar []
  (mk-foo))

(defn mk-baz []
  (binding [mk-foo #(do (fn [] 36))]
    (mk-bar)))

((mk-baz))
;=> 36

((mk-bar))
;=> 42