Coal stratum (4)
I believe the answer is:
seam
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coal stratum</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)