Enthusiasts rising in shock (4)
I believe the answer is:
stun
'shock' is the definition.
(both can mean to surprise)
'enthusiasts rising' is the wordplay.
'enthusiasts' becomes 'nuts' (nut can mean an enthusiast or fan).
'rising' says the letters should be written in reverse (going up in a down clue).
'nuts' backwards is 'STUN'.
'in' acts as a link.
(Other definitions for stun that I've seen before include "Anaesthetise" , "Shock into silence" , "Astound; render senseless" , "Stupefy - daze" , "Bowl over" .)