An urgent call about top-quality fireproofing (8)
I believe the answer is:
asbestos
'fireproofing' is the definition.
(thesaurus)
'an urgent call about top-quality' is the wordplay.
'an' becomes 'a'.
'urgent call' becomes 'SOS' (an SOS message sent in an emergency).
'about' indicates putting letters inside.
'top quality' becomes 'best' (I've seen this in another clue).
'sos' enclosing 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Fire-resistant mineral" , "Insulating mineral" , "Fibrous material, doesn't burn" , "Firewall material" , "Once used for safety, it proved dangerous" .)