Form of green bananas (5)
I believe the answer is:
genre
'form' is the definition.
(genre is a kind of form)
'green bananas' is the wordplay.
'bananas' indicates anagramming the letters (informal term for mad).
'green' anagrammed gives 'GENRE'.
'of' is the link.
(Other definitions for genre that I've seen before include "expressive style of music" , "Green (anag)" , "Particular style of art or literature, may be green" , "Literary type, species" , "Category (of art)" .)