Vein of coal (4)
I believe the answer is:
seam
'vein of coal' 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 "Stitched joint" , "Unfasten" , "Join in material - layer of coal" , "Join - stratum of ore" , "Stratum (of a mineral)" .)