A call for help about finest insulating material (8)
I believe the answer is:
asbestos
'insulating material' is the definition.
(material formerly used as insulation)
'a call for help about finest' is the wordplay.
'call for help' becomes 'sos' (SOS distress signal).
'about' 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 "Boss set a non-burning material up" , "Insulating mineral" , "we now know to be dangerous" , "Fire-retardant material" , "Fire defence" .)