A girl to spoil, that is (5)
I believe the answer is:
marie
'a girl' is the definition.
'spoil that is' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'mar'+'ie'='MARIE'
'to' is the link.
(Other definitions for marie that I've seen before include "Queen, guillotined in 1793" , "-- Curie, scientist" , "French queen, d.1793" , "girl" , "- Lloyd, English music- hall entertainer" .)