Here’s my attempt:
(defn gcd [a b]
  (if (= b 0)
    a
    (recur b (rem a b))))

(defn totient [n]
  (if (<= n 1)
    1
    (loop [i (- n 1) a 0]
      (if (> i 0)
        (recur (- i 1) (+ a (if (= (gcd n i) 1) 1 0)))
        a))))