Oil-bearing rock (5)
I believe the answer is:
shale
'oil-bearing rock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for shale that I've seen before include "Stratified rock" , "Stone that splits easily into fine pieces" , "Soft sedimentary rock" , "Type of soft rock" , "Easily split layered rock" .)