Fuel cartel (4)
I believe the answer is:
'fuel cartel' 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 "Group of petroleum producing countries, formed in 1961" , "Organisation intended to control the production and sale of petroleum" , "Cartel of oil-producing countries" , "Group of countries with a common oil policy" , "Group of oil-producing countries" .)