Opt for oil processing as a minister's responsibility (9)
I believe the answer is:
portfolio
'a minister's responsibility' is the definition.
(I've seen this before)
'opt for oil processing' is the wordplay.
'processing' indicates an anagram.
'opt'+'for'+'oil'='optforoil'
'optforoil' is an anagram of 'PORTFOLIO'.
'as' acts as a link.
(Other definitions for portfolio that I've seen before include "Range of investments" , "governmental responsibility" , "Set of collected pieces of creative work" , "artist's sample work" , "Set of investments held by one person" .)