Bananas aren't nice fruit (9)
I believe the answer is:
nectarine
'fruit' is the definition.
(nectarine is a kind of fruit)
'bananas aren't nice' is the wordplay.
'bananas' indicates anagramming the letters (bananas can informally mean crazy).
'arent'+'nice'='arentnice'
'arentnice' anagrammed gives 'NECTARINE'.
(Other definitions for nectarine that I've seen before include "Kind of peach with thin skin and no down" , "Trace nine to a kind of peach" , "Smooth-skinned fruit" , "Entrance I make with thin-skinned peach" , "type of tree" .)