It's crazy, when upset, to cause stupefaction (4)
I believe the answer is:
stun
'to cause stupefaction' is the definition.
I can't judge whether this defines the answer.
'it's crazy when upset' is the wordplay.
'it's crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'when upset' shows that the letters should be reversed in order (in a down clue - set, written, up).
'nuts' reversed gives 'STUN'.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Anaesthetise" , "Numb with nuts" , "Knock into a semi-conscious state" , "Daze or bewilder" , "Daze, knock senseless" .)