Spoil a horse, docking its tail (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'a horse docking its tail' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
The remaining letters 'mr' is a valid word which might be clued in a way I don't understand.
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 "Spoil or deface" , "'Damage, impair (3)'" , "Cause harm" , "Ruin" , "Detract from" .)