Source of oil put in some butter for protection (7)
I believe the answer is:
parapet
'for protection' is the definition.
(a parapet is a protective feature)
'source of oil put in some butter' is the wordplay.
'source of oil' becomes 'rape' (as in rapeseed oil).
'put in' is an insertion indicator.
'some butter' becomes 'pat' (a pat can mean a lump of butter).
'rape' put into 'pat' is 'PARAPET'.
(Other definitions for parapet that I've seen before include "wall used for protection" , "keep overlooking this?" , "Wall's top" , "Wall at roof's edge" , "wall, protective one" .)