A call for help when holding first-rate insulating material (8)
I believe the answer is:
asbestos
'insulating material' is the definition.
(material formerly used as insulation)
'a call for help when holding first-rate' is the wordplay.
'call for help' becomes 'sos' (I've seen this before).
'when holding' indicates putting letters inside.
'first-rate' becomes 'best' (I've seen this in another clue).
'sos' placed around 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "we now know to be dangerous" , "dangerous stuff" , "Protective fabric" , "Heat-resistant fibrous mineral" , "condemned building material" .)