Fruitcake can't use bananas (7)
I believe the answer is:
nutcase
'fruitcake' is the definition.
(I've seen this before)
'can't use bananas' is the wordplay.
'bananas' indicates an anagram (informal term for mad).
'cant'+'use'='cantuse'
'cantuse' with letters rearranged gives 'NUTCASE'.
(Other definitions for nutcase that I've seen before include "fool" , "Crank" , "Loony" , "Psycho" , "Eccentric" .)