Oil source (5)
I believe the answer is:
'oil source' 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 "Fine pieces of stone on leash" , "Layered sedimentary rock" , "Clayey rock" , "Stone that splits easily into fine pieces" , "Soft rock; heals (anag.)" .)