Form of green composition (5)
I believe the answer is:
genre
'form' is the definition.
(genre is a kind of form)
'green composition' is the wordplay.
'composition' indicates an anagram.
'green' is an anagram of 'GENRE'.
'of' acts as a link.
(Other definitions for genre that I've seen before include "Green (anag)" , "Style of work" , "Style/category of art" , "Brand" , "Literary type, species" .)