Building material in East London obtains top rating (8)
I believe the answer is:
asbestos
'building material' is the definition.
I can't explain the remainder of the clue.
(Other definitions for asbestos that I've seen before include "we now know to be dangerous" , "Fire-resistant material dangerous to health" , "Heat-resistant material" , "Survivor of heat" , "Fire-retardant material" .)