Coal seam (4)
I believe the answer is:
vein
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coal seam</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for vein that I've seen before include "Vessel carrying blood towards the heart" , "Vessel for claret" , "part of a vital system" , "Blood-carrying vessel" , "Tube that carries blood to the heart" .)