Left smear of oil in minister's office (9)
I believe the answer is:
portfolio
'minister's office' is the definition.
(I've seen this before)
'left smear of oil' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'smear' is an anagram indicator (I've seen 'smeared' mean this).
'of'+'oil'='ofoil'
'ofoil' with letters rearranged gives 'folio'.
'port'+'folio'='PORTFOLIO'
'in' acts as a link.
(Other definitions for portfolio that I've seen before include "Case for loose paper, drawings etc" , "Shares" , "What Secretary of State was appointed to" , "Collection of papers" , "Set of papers" .)