Aromatic leaves spoil sweet butter (8)
I believe the answer is:
marjoram
'aromatic leaves' is the definition.
'spoil sweet butter' is the wordplay.
'spoil' becomes 'mar' (I've seen this before).
'sweet' becomes 'jo' (old-fashioned term of endearment).
'butter' becomes 'ram' (I've seen this before).
'mar'+'jo'+'ram'='MARJORAM'
(Other definitions for marjoram that I've seen before include "Food seasoning also called oregano" , "Herb used in soups, stews etc" , "Arm major (anag)" , "Kind of herb" , "found with Rosemary in the kitchen?" .)