Female horse losing tail is hurt (3)
I believe the answer is:
mar
'is hurt' is the definition.
(both can mean to harm)
'female horse losing tail' is the wordplay.
'female horse' becomes 'mare' (synonyms).
'losing tail' means to remove the last letter.
'mare' with its final letter taken away is 'MAR'.
(Other definitions for mar that I've seen before include "Spoil, damage" , "Ram into and spoil" , "Blemish" , "Blight" , "Oh dear!" .)