Spoil European horse (4)
I believe the answer is:
mare
'horse' is the definition.
(mare is a kind of horse)
'spoil european' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'european' becomes 'E' (abbreviation e.g. EU).
'mar'+'e'='MARE'
(Other definitions for mare that I've seen before include "Horse; lunar sea" , "Nag" , "Ream (anag)" , "A night-time strangler" , "Female quadruped" .)