A call for assistance carrying cream building material (8)
I believe the answer is:
asbestos
'building material' is the definition.
'a call for assistance carrying cream' is the wordplay.
'call for assistance' becomes 'sos'.
'carrying' means one lot of letters goes inside another.
'cream' becomes 'best' (I've seen this before).
'sos' going around 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Heat-resistant fibrous mineral" , "'Fireproof substance, dangerous to health (8)'" , "dangerous dust" , "Fire-retardant material" , "Protective fabric" .)