Horse without tail is wreck (3)
I believe the answer is:
mar
'wreck' is the definition.
(I've seen this in another clue)
'horse without tail' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'without tail' means to remove the last letter.
'mare' with its final letter taken away is 'MAR'.
'is' acts as a link.
(Other definitions for mar that I've seen before include "Spoil something" , "Detract from" , "Cause harm" , "Spoil or deface" , "Spoil stuff" .)