Bananas, oddly enough (4)
I believe the answer is:
nuts
'bananas' is the definition.
(as in crazy)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nuts that I've seen before include "Eccentric; coal lumps" , "deprived of one's wits" , "Hard-shelled fruit" , "Fruit - bats" , "Mad - hard fruits" .)