Fiddle by mixing oil in French wine (6)
I believe the answer is:
violin
'fiddle' is the definition.
(I know that fiddle can be written as violin)
'mixing oil in french wine' is the wordplay.
'mixing' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'french wine' becomes 'vin' ('wine' in French).
'oil' anagrammed gives 'oli'.
'oli' put inside 'vin' is 'VIOLIN'.
'by' is the link.
(Other definitions for violin that I've seen before include "It'a played with a bow" , "Menuhin's instrument" , "Nigel Kennedy instrument" , "A stringed musical instrument, say a Stradivarius" , "Instrument with bow" .)