Knock senseless (4)
I believe the answer is:
stun
'knock senseless' is the definition.
(I've seen this before)
This is all the clue.
(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" .)