Fashion line? (4)
I believe the answer is:
'fashion 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 "Line of sewing - stratum of coal" , "Join - stratum of ore" , "Sewn join" , "Join in material - layer of coal" , "Unfasten" .)