Curse as horse brought to grass (4)
I believe the answer is:
oath
'curse' is the definition.
(I know that curse can be written as oath)
'horse brought to grass' is the wordplay.
'horse' becomes 'h' (synonyms).
'brought to' is a charade indicator (letters next to each other).
'grass' becomes 'oat' (I've seen this in other clues).
'h' put after 'oat' is 'OATH'.
'as' is the link.
(Other definitions for oath that I've seen before include "darn, for example" , "Imprecation" , "blasphemy" , "It could be ruddy when minced" , "word intended?" .)