FizzBuzz - officially now in CLISP
Do not recompute is-multiple repeatedly by either binding the value:

(defun num-action (i)
  (let ((i3 (is-multiple i 3))
        (i5 (is-multiple i 5)))
    (cond ((and i3 i5) (print "FizzBuzz"))
          (i3 (print "Fizz"))
          (i5 (print "Buzz"))
          (T (print i)))))


or by using if:

(defun num-action (i)
  (if (is-multiple i 3)
      (if (is-multiple i 5)
          (print "FizzBuzz")
          (print "Fizz"))
      (if (is-multiple i 5)
          (print "Buzz")
          (print i))))


PS. please fix indentation