Shock over bananas (4)
I believe the answer is:
nuts
'bananas' is the definition.
(as in crazy)
'shock over' is the wordplay.
'shock' becomes 'stun' (synonyms).
'over' says the letters should be written in reverse.
'stun' in reverse letter order is 'NUTS'.
(Other definitions for nuts that I've seen before include "Crackers - or what crackers are required for?" , "One eats these" , "Nibbles" , "Crazy; food" , "Metal fasteners" .)