Something bad in grass, not good (3)
I believe the answer is:
sin
'something bad' is the definition.
(something morally bad)
'in grass not good' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
A single letter 's' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for sin that I've seen before include "Behave badly" , "Pride, say" , "Immorality" , "Breach of moral code" , "grave offence" .)