Spoil Scottish sweetheart with butter and herb (8)
I believe the answer is:
marjoram
'herb' is the definition.
(marjoram is a kind of herb)
'spoil scottish sweetheart with butter' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'scottish sweetheart' becomes 'jo' (Scottish term for darling or sweetheart).
'with' means one lot of letters go next to another.
'butter' becomes 'ram' (I've seen this before).
'mar'+'jo'+'ram'='MARJORAM'
'and' acts as a link.
(Other definitions for marjoram that I've seen before include "Herb used in cooking - a Mr Major (anag)" , "Aromatic mint relative" , "Aromatic herb" , "pot plant" , "found with Rosemary in the kitchen?" .)