Curses horses at the front getting stuck into horse-feed (5)
I believe the answer is:
oaths
'curses' is the definition.
(I know this)
'horses at the front getting stuck into horse-feed' is the wordplay.
'at the front' says to take the initial letters.
'getting stuck into' means one lot of letters goes inside another.
'horse feed' becomes 'oats' (I've seen this in another clue).
The initial letter of 'horses' is 'h'.
'h' put into 'oats' is 'OATHS'.
(Other definitions for oaths that I've seen before include "truths may be established by them" , "professions of loyalty" , "Expletives" , "Solemn vows" , "four-letter words" .)