Heat-resistant mineral (8)
I believe the answer is:
asbestos
'heat-resistant mineral' is the definition.
(I've seen this before)
This is all the clue.
(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" .)