Straight line’s drawn in a fine green (5)
I believe the answer is:
leafy
'green' is the definition.
(I've seen this before)
'straight line's drawn in a fine' is the wordplay.
'straight line' becomes 'ley' (as in ley lines).
'drawn in' indicates putting letters inside (inserted letters are drawn in).
'fine' becomes 'f' (abbreviation).
'a'+'f'='af'
'ley' placed around 'af' is 'LEAFY'.
(Other definitions for leafy that I've seen before include "Like the better class suburbs?" , "Full of foliage" , "Epithet applied to suburbs" , "Green" , "Like nice suburbs" .)