Soft, sedimentary rock (5)
I believe the answer is:
shale
'soft sedimentary rock' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for shale that I've seen before include "Oil-bearing rock" , "Type of soft rock" , "Stratified rock" , "Fine pieces of stone on leash" , "Stone that splits easily into fine pieces" .)