There is no explicit return value because the functions aren't functions: they are values, thus the result of the algorithm is bound to the value that is bound to the function.

When you write:

(defn square [x]

     (* x x))

You are really writing:

(def square

    (fn [x]

        (* x x)))

and x * x is bound to the value of square. There would be no logical reason to have an explicit return value here.