If you're trying to make something in pure Lisp, your odds of failure are less if you take an existing hashing algorithm (e.g. SHA-256) and just iterate it a bunch of times. Ironclad has SHA-256, so this is really easy:

    (defun slow-hash (password salt &key (iterations 10000))
      "Produces a 256-bit hashed value of password and salt, slowly. Uses
      a tweakable number of iterations, which should not be less than
      1000, and which defaults to 10000."
      (let ((hash (ironclad:make-digest :sha256)))
        ;; First, hash the salt and password
        (ironclad:update-digest hash
           (ironclad:ascii-string-to-byte-array salt))
        (ironclad:update-digest hash
           (ironclad:ascii-string-to-byte-array password))
        ;; Repeatedly hash the hash, to slow things down
        (dotimes (x iterations)
          (ironclad:update-digest hash (ironclad:produce-digest hash)))
        (ironclad:produce-digest hash)))