Oil group (4)
I believe the answer is:
opec
'oil group' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opec that I've seen before include "International oil-producing cartel" , "Group of petroleum producing countries, formed in 1961" , "Cartel of oil-producing countries" , "[CRUDE OIL] suppliers?" , "Organisation intended to control the production and sale of petroleum" .)