You don't - the clipboard is just an area of RAM set aside to use as a temporary holding place. If you restart the phone (or turn it off, then turn it on), all of RAM, including the clipboard, is cleared. If you want to "clear: it of something important, just copy some junk and that will replace what was in there before.
Edit:
Forget I said anything - Huawei evidently doesn't use the stock Android clipboard.
Thanks, Tsepz.