Form of green composition (5)
I believe the answer is:
genre
'form' is the definition.
(I know that genre is a type of form)
'green composition' is the wordplay.
'composition' is an anagram indicator (letters arranged or composed in a different order).
'green' with letters rearranged gives 'GENRE'.
'of' is the link.
(Other definitions for genre that I've seen before include "Green (anag)" , "Style of work" , "Style/category of art" , "Brand" , "Literary type, species" .)