Crazy upset brings shock (4)
I believe the answer is:
stun
'shock' is the definition.
(both can mean to surprise)
'crazy upset' is the wordplay.
'crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'upset' is a reversal indicator.
'nuts' reversed gives 'STUN'.
'brings' is the link.
(Other definitions for stun that I've seen before include "Daze or bewilder" , "Daze, paralyse" , "Daze, render semi-conscious" , "Bowl over" , "Shock or astonish" .)