A rough description of oil (5)
I believe the answer is:
crude
This clue could be a double definition.
'a rough' is the first definition.
(I've seen this before)
'description of oil' is the second definition.
'crude' can be an answer for 'oil' (crude is a kind of oil). I'm unsure of the 'description of' bit.
Can you help me to learn more?
(Other definitions for crude that I've seen before include "Oil; rough" , "Rude; raw" , "Petroleum" , "Coarse and unrefined" , "Not refined, oil say" .)