State, area involved in distribution of oil in USA (9)
I believe the answer is:
louisiana
'state' is the definition.
(Louisiana is an example)
'area involved in distribution of oil in usa' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'involved in' indicates putting letters inside.
'distribution of' indicates anagramming the letters.
'oil'+'in'+'usa'='oilinusa'
'oilinusa' with letters rearranged gives 'louisina'.
'a' going into 'louisina' is 'LOUISIANA'.
(Other definitions for louisiana that I've seen before include "US state, capital Baton Rouge" , "Southern United State on the Gulf of Mexico" , "American state, capital Baton Rouge" , "IOU a snail over in the US state" , "that's near Mississippi" .)