Hazardous material? Most desirable to enter a call for help (8)
I believe the answer is:
asbestos
'hazardous material?' is the definition.
(I know that asbestos is a hazardous material)
'most desirable to enter a call for help' is the wordplay.
'most desirable' becomes 'best' (similar in meaning).
'to enter' is an insertion indicator.
'call for help' becomes 'sos' (SOS distress signal).
'a'+'sos'='asos'
'best' placed inside 'asos' is 'ASBESTOS'.
(Other definitions for asbestos that I've seen before include "I might end up lagging" , "Heat-resistant fibre" , "'Fireproof substance, dangerous to health (8)'" , "we now know to be dangerous" , "Boss set a non-burning material up" .)