Surprise or 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 before)
(Other definitions for stun that I've seen before include "Bowl over" , "Daze with nuts" , "Daze or bewilder" , "Knock unconscious" , "cause amazement" .)