Sewn line (4)
I believe the answer is:
seam
'sewn line' 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 "Fabric joint" , "Join in knitted garment" , "Underground layer of mineral" , "Line of sewing - stratum of coal" , "Stratum of ore or coal" .)