Amaze fanatics on return (4)
I believe the answer is:
stun
'amaze' is the definition.
(to stun is to amaze or shock)
'fanatics on return' is the wordplay.
'fanatics' becomes 'nuts' (nut can mean an enthusiast or fanatic).
'on return' is a reversal indicator.
'nuts' written backwards gives 'STUN'.
(Other definitions for stun that I've seen before include "Shock, flabbergast" , "Knock unconscious" , "Daze, overwhelm" , "Take aback" , "Daze, paralyse" .)