Spoil English horse (4)
I believe the answer is:
mare
'horse' is the definition.
(I know that mare is a type of horse)
'spoil english' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'english' becomes 'E' (abbreviation).
'mar'+'e'='MARE'
(Other definitions for mare that I've seen before include "Adult female horse" , "For classicists, sea" , "Female quadruped" , "Ream (anag)" , "A night-time strangler" .)