Promise of heroin after grass... (4)
I believe the answer is:
oath
'promise' is the definition.
(thesaurus)
'heroin after grass' is the wordplay.
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'after' means one lot of letters go next to another.
'grass' becomes 'oat' (I've seen this in other clues**).
'h' after 'oat' is 'OATH'.
'of' is the link.
(Other definitions for oath that I've seen before include "A curse or solemn declaration" , "Courtroom promise" , "What frustrated people say" , "'Profane, or solemn expression (4)'" , "Solemn promise to tell the truth or be loyal" .)