Small bag fed to horse results in heavy defeat (8)
I believe the answer is:
massacre
'heavy defeat' is the definition.
(I've seen this in another clue)
'small bag fed to horse' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bag' becomes 'sac' (a small bag).
'fed to' indicates putting letters inside.
'horse' becomes 'mare' (mare is a kind of horse).
's'+'sac'='ssac'
'ssac' put into 'mare' is 'MASSACRE'.
'results in' is the link.
(Other definitions for massacre that I've seen before include "Complete defeat" , "Heavily defeat" , "Widespread killing" , "bloody victory" , "Destroy" .)