Officer to spoil sweetheart inside (5)
I believe the answer is:
major
'officer' is the definition.
(type of military officer)
'spoil sweetheart inside' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'sweetheart' becomes 'jo' (Scottish term for darling or sweetheart).
'inside' means one lot of letters goes inside another.
'mar' going around 'jo' is 'MAJOR'.
'to' acts as a link.
(Other definitions for major that I've seen before include "Large" , "Significant; rank" , "Important; rank" , "Principal" , "Main course" .)