Knock senseless (4)
I believe the answer is:
stun
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">knock senseless</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for stun that I've seen before include "Daze, overwhelm" , "Daze, scatter senses" , "Daze with nuts" , "Knock unconscious" , "deliver a knock-out blow" .)