Fine-grained sedimentary rock (5)
I believe the answer is:
shale
'fine-grained sedimentary rock' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for shale that I've seen before include "Oil-rich rock" , "Stone that splits easily into fine pieces" , "Layered sedimentary rock" , "Clayey rock" , "Type of soft rock" .)