Hard coal (10)
I believe the answer is:
anthracite
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hard coal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for anthracite that I've seen before include "Hard variety of coal" , "High-grade hard coal" , "kind of fuel" , "Hard type of coal" .)