Wealthy sort one in mineral oil refinement? (11)
I believe the answer is:
millionaire
'wealthy sort' is the definition.
(I know this)
'one in mineral oil refinement?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'refinement?' is an anagram indicator (letters must be refined).
'mineral'+'oil'='mineraloil'
'mineraloil' with letters rearranged gives 'millionare'.
'i' going within 'millionare' is 'MILLIONAIRE'.
(Other definitions for millionaire that I've seen before include "Rich fellow" , "Airline limo (anag) - rich person" , "Very wealthy person" , "See 14" .)