Surprise as fans returning (4)
I believe the answer is:
stun
'surprise' is the definition.
(stunning is a kind of surprising)
'fans returning' is the wordplay.
'fans' becomes 'nuts' (both can mean enthusiasts).
'returning' is a reversal indicator.
'nuts' in reverse letter order is 'STUN'.
'as' is the link.
(Other definitions for stun that I've seen before include "Daze, concuss" , "Render insensitive" , "'Shock, daze (4)'" , "Shock or astonish" , "what a cracker may do?" .)