Vein of coal or ore (4)

I believe the answer is:

seam

(Other definitions for seam that I've seen before include "Line of sewing - stratum of coal" , "The line where two edges are stitched together" , "Line of join" , "Join in knitted garment" , "A stitched join in a garment" .)

I've seen this clue in The Times.

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