Spoil a horse, docking the tail (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'a horse docking the tail' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
The remaining letters 'mr' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mar that I've seen before include "wreck" , "Spoil or deface" , "Make imperfect" , "Spoil stuff" , "Cause harm" .)