Surprise as lunatics returning (4)
I believe the answer is:
Here is my best explanation:
'surprise' is the definition.
(stunning is a kind of surprising)
'lunatics returning' is the subsidiary indication.
'lunatics' becomes 'nuts'.
'returning' is a reversal indicator.
'nuts' backwards is 'stun'.
'as' is the link.
(Other definitions for stun that I've seen before include "Leave speechless", "Amaze; knock senseless", "Knock unconscious", "Daze, scatter senses", "deliver a knock-out blow", "Render temporarily senseless", "Astonish", "Taser", "Confound" and "Make senseless or dizzy with a blow".)