Surprise as lunatics returning (4)
I believe the answer is:
stun
'surprise' is the definition.
(stunning is a kind of surprising)
'lunatics returning' is the wordplay.
'lunatics' becomes 'nuts'.
'returning' shows that the letters should be reversed in order.
'nuts' reversed gives 'STUN'.
'as' acts as a link.
(Other definitions for stun that I've seen before include "deliver a knock-out blow" , "Nuts (anag)" , "what a cracker may do?" , "Shock, flabbergast" , "Bowl over" .)