Grass may be evil -- and good (4)
I believe the answer is:
sing
'grass' is the definition.
(I've seen this in other clues)
'evil and good' is the wordplay.
'evil' becomes 'sin' (I've seen this before).
'and' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'sin'+'g'='SING'
'may be' is the link.
(Other definitions for sing that I've seen before include "Blow the whistle" , "Make melodious sounds with voice" , "Celebrate" , "Perform (a song)" , "Vocalise musically" .)