Shock or overwhelm (4)
I believe the answer is:
stun
This clue is a double definition.
'shock' is the first definition.
(both can mean to surprise)
'overwhelm' is the second definition.
(I've seen this in another clue)
(Other definitions for stun that I've seen before include "Stupefy - daze" , "Render temporarily senseless" , "deliver a knock-out blow" , "Render insensitive" , "Bowl over" .)