Surprise, astound (4)
I believe the answer is:
stun
I believe this is a double definition.
'surprise' is the first definition.
(stunning is a kind of surprising)
'astound' is the second definition.
(I've seen this in another clue)
(Other definitions for stun that I've seen before include "Daze, scatter senses" , "Daze, render semi-conscious" , "Knock into a semi-conscious state" , "cause amazement" , "Astound; render senseless" .)