Most common sedimentary rock (5)
I believe the answer is:
shale
'most common sedimentary 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 "Soft sedimentary rock" , "Clayey rock" , "Stone that splits easily into fine pieces" , "Type of soft rock" , "Oil-rich rock" .)