Answer cry for help to protect finest building material (8)
I believe the answer is:
asbestos
'building material' is the definition.
'answer cry for help to protect finest' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'cry for help' becomes 'sos'.
'to protect' is an insertion indicator.
'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 "dangerous dust" , "Fibrous material, doesn't burn" , "Fire-resistant material, a danger to health" , "retardant" , "Incombustible material" .)