Answer SOS to protect finest building material (8)
I believe the answer is:
asbestos
'building material' is the definition.
'answer sos to protect finest' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'to protect' indicates putting letters inside.
'finest' becomes 'best' (similar in meaning).
'sos' going around 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Heat-resistant material" , "I might end up lagging" , "retardant" , "Fire-resistant material, injurious to health" , "Fireproof material, now known to be dangerous" .)