If clear-input does not make sense for input-stream, then clear-input does nothing.

;; The exact I/O behavior of this example might vary from implementation
;; to implementation depending on the kind of interactive buffering that
;; occurs.  (The call to SLEEP here is intended to help even out the 
;; differences in implementations which do not do line-at-a-time buffering.)

(defun read-sleepily (&optional (clear-p nil) (zzz 0))
  (list (progn (print '>) (read))
        ;; Note that input typed within the first ZZZ seconds 
        ;; will be discarded.
        (progn (print '>) 
               (if zzz (sleep zzz))
               (print '>>)
               (if clear-p (clear-input))
               (read))))

(read-sleepily)
> 10
>
>> 20
 (10 20)

(read-sleepily t)
> 10
>
>> 20
 (10 20)

(read-sleepily t 10)
> 10
> 20  ; Some implementations won't echo typeahead here.
>> 30
 (10 30)