(defn flat?
  "true if there is no sequences"
  [seq]
  (not-any? (fn [x] (isa? (type x) java.util.List)) seq))

(defn my-flatten
  "Returns flat sequence"
  [seq]
  (if (isa? (type seq) java.util.List)
    (if (flat? seq)
      seq
      (apply concat (map flatten seq)))
    (list seq)))

    Test:

    (my-flatten [1,[7,0],2,[3,8, [7]],4])