Instrument for oil producer in Virginia (5)
I believe the answer is:
viola
'instrument' is the definition.
(I know that viola is a type of instrument)
'oil producer in virginia' is the wordplay.
'producer' is an anagram indicator.
'in' means one lot of letters goes inside another.
'virginia' becomes 'va' (US state abbreviation).
'oil' is an anagram of 'iol'.
'iol' going into 'va' is 'VIOLA'.
'for' acts as a link.
(Other definitions for viola that I've seen before include "Shakespearean character" , "Pansy-like flower" , "Page" , "Flower -- stringed instrument" , "Sebastian's twin (Twelfth Night)" .)