Does this base-10 number contain the digit 7? (a career fair question solution)

contains7.clj

(defn contains-7? [number]
  (if (zero? number) false
    (letfn [(seven? [n]
                    (== 7 n))
            (one-digit? [n]
                        (< n 10))
            (leading-digit [n]
                (int
                  (if (one-digit? n)
                    n
                    (leading-digit (/ n 10)))))]
      (let [place-holder-digits (int (Math/log10 number))
            ld (leading-digit number)]
        (if (seven? ld) true
            (recur (- number (* ld (Math/pow 10 place-holder-digits)))))))))