Damage horse, having tail docked (3)
I believe the answer is:
mar
'damage' is the definition.
(to mar is to spoil or damage)
'horse having tail docked' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'having tail docked' means to remove the last letter.
'mare' with its final letter removed is 'MAR'.
(Other definitions for mar that I've seen before include "Detract from" , "Blight" , "Oh dear!" , "hurt" , "Ruin" .)