Fanatic's back flips are stupefying (4)
I believe the answer is:
stun
'flips are stupefying' is the definition.
I can't judge whether this definition defines the answer.
'fanatic's back' is the wordplay.
'fanatic's' becomes 'nut's'.
'back' says the letters should be written backwards.
'nuts' backwards is 'STUN'.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Daze, paralyse" , "Render semi-conscious with blow" , "Knock cold" , "Put out for the count" , "Surprise greatly or daze" .)