Crude group? (4)
I believe the answer is:
'crude group?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opec that I've seen before include "Group of petroleum producing countries, formed in 1961" , "International oil-producing cartel" , "price-fixers?" , "Group of countries with a common oil policy" , "Organisation intended to control the production and sale of petroleum" .)