Heat-resistant mineral (8)
I believe the answer is:
asbestos
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">heat-resistant mineral</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for asbestos that I've seen before include "Fire-resistant material, a danger to health" , "Fire-resistant material dangerous to health" , "Fire-resistant material, injurious to health" , "Once used for safety, it proved dangerous" , "Old fire-retardant" .)