Coal stratum (4)
I believe the answer is:
seam
'coal stratum' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seam that I've seen before include "Unfasten" , "Joint between two edges" , "The line where two edges are stitched together" , "Join in knitted garment" , "Fabric join" .)