Surprise enthusiasts making a comeback (4)
I believe the answer is:
stun
'surprise' is the definition.
(stunning is a kind of surprising)
'enthusiasts making a comeback' is the wordplay.
'enthusiasts' becomes 'nuts' (nut can mean an enthusiast or fan).
'making a comeback' says the letters should be written in reverse.
'nuts' back-to-front is 'STUN'.
(Other definitions for stun that I've seen before include "Make dazed or numb" , "Confound" , "Render temporarily senseless" , "Shock, flabbergast" , "Knock cold" .)