Harm horse, docking tail (3)
I believe the answer is:
mar
'harm' is the definition.
(I've seen this before)
'horse docking tail' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'docking tail' means to remove the last letter.
'mare' with its last letter taken off is 'MAR'.
(Other definitions for mar that I've seen before include "Oh dear!" , "Spoil something" , "Disfigure" , "hurt" , "Spoil or deface" .)