Some butter pastry shortened with occasional doses of plain oil (9)
I believe the answer is:
patchouli
'plain oil' is the definition.
'patchouli' can be an answer for 'oil' (I've seen this before). I am unsure of the 'plain' bit.
'some butter pastry shortened with occasional doses' is the wordplay.
'some butter' becomes 'pat' (a pat can mean a lump of butter).
'pastry' becomes 'chou' (chou is a kind of pastry).
'shortened with occasional doses' becomes 'li' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pat'+'chou'+'li'='PATCHOULI'
'of' is the link.
Can you help me to learn more?
(Other definitions for patchouli that I've seen before include "scented oil" , "Aromatic oil" , "essential oil" , "County (abbrev.); weapons" , "scented shrub" .)