Recurved bananas make quite an impact (4)
I believe the answer is:
stun
'make quite an impact' is the definition.
'recurved bananas' is the wordplay.
'recurved' is a reversal indicator.
'bananas' becomes 'nuts' (as in crazy).
'nuts' in reverse letter order is 'STUN'.
(Other definitions for stun that I've seen before include "Daze, stupify" , "Knock unconscious" , "Make dazed or numb" , "Daze, overwhelm" , "'Shock, daze (4)'" .)