with-redefs sample

(def best-player "Xavi")

(defn greeting 
	[]
	(prn (format "Hi %s is the best player!", best-player)))

(defn greeting-mock
	[]
	(prn (format "Hi. Now %s rocks!", best-player)))

(defn foo
	[]
	(prn "Do stuff")
	(greeting)
	(prn "Do stuff"))

(with-redefs [best-player "Messi" 
	          greeting greeting-mock]
	          (foo))