Spoil horse with no tail (3)
I believe the answer is:
mar
'spoil' is the definition.
(synonyms)
'horse with no tail' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'with no tail' means to remove the last letter.
'mare' with its last letter taken away is 'MAR'.
(Other definitions for mar that I've seen before include "Spoil or deface" , "Damage; month (abbr.)" , "'Impair, spoil (3)'" , "Spoil, damage" , "Spanish main?" .)