Fiddle mixing oil into French wine (6)
I believe the answer is:
violin
'fiddle' is the definition.
('violin' can be a synonym of 'fiddle')
'mixing oil into french wine' is the wordplay.
'mixing' indicates an anagram.
'into' indicates putting letters inside.
'french wine' becomes 'vin' (I've seen this before).
'oil' anagrammed gives 'oli'.
'oli' placed inside 'vin' is 'VIOLIN'.
(Other definitions for violin that I've seen before include "An elevated fiddle" , "Menuhin's instrument" , "Instrument with four strings" , "It'a played with a bow" , "A stringed musical instrument, say a Stradivarius" .)