Line of stitching or layer of ore (4)
I believe the answer is:
seam
I believe this clue is a double definition.
'line of stitching' is the first definition.
(I've seen this in another clue)
'layer of ore' is the second definition.
(I've seen this in another clue)
(Other definitions for seam that I've seen before include "Stratum of ore or coal" , "Sewn join" , "A stitched join in a garment" , "Line of sewing - stratum of coal" , "Joint between two edges" .)