Fanatics stagger back (4)
I believe the answer is:
nuts
'fanatics' is the definition.
(nut can mean an enthusiast or fanatic)
'stagger back' is the wordplay.
'stagger' becomes 'stun' (both can mean to shock).
'back' shows that the letters should be reversed in order.
'stun' written backwards gives 'NUTS'.
(Other definitions for nuts that I've seen before include "Crazy; food" , "Got a screw loose" , "Metal fasteners" , "Delivers 24 [HEADBUTT]" , "off one's trolley" .)