Oil stirred into French wine is a fiddle (6)
I believe the answer is:
violin
'a fiddle' is the definition.
(I know that violin is a fiddle)
'oil stirred into french wine' is the wordplay.
'stirred' is an anagram indicator.
'into' means one lot of letters goes inside another.
'french wine' becomes 'vin' ('wine' in French).
'oil' is an anagram of 'oli'.
'oli' put inside 'vin' is 'VIOLIN'.
'is' acts as a link.
(Other definitions for violin that I've seen before include "Bowed instrument" , "A stringed musical instrument, say a Stradivarius" , "Menuhin's instrument" , "Instrument with four strings" , "An elevated fiddle" .)