Layered rock (5)

I believe the answer is:

shale

'layered 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" , "Soft rock; heals (anag.)" , "Easily-split sedimentary rock" , "Fine pieces of stone on leash" , "Layered sedimentary rock" .)

I've seen this clue in the Penny Press and The New York Times.

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