A call for help about finest incombustible material (8)
I believe the answer is:
asbestos
'incombustible material' is the definition.
(I've seen this before)
'a call for help about finest' is the wordplay.
'call for help' becomes 'sos' (I've seen this before).
'about' means one lot of letters goes inside another.
'finest' becomes 'best' ('best' can be similar in meaning to 'finest').
'sos' placed around 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Heat-resistant fibrous mineral" , "dangerous stuff" , "Fireproof material, dangerous to health" , "dangerous dust" , "you can't inflame this" .)