Rejected bananas are overpowering (4)
I believe the answer is:
stun
'are overpowering' is the definition.
('they are overpowering' could mean 'they stun')
'rejected bananas' is the wordplay.
'rejected' shows that the letters should be reversed in order.
'bananas' becomes 'nuts' (as in crazy).
'nuts' backwards is 'STUN'.
(Other definitions for stun that I've seen before include "Daze, render semi-conscious" , "Make senseless or dizzy with a blow" , "Surprise greatly, knock someone''s socks off" , "Astound; render senseless" , "Render unconscious" .)