A distress call about most dangerous material (8)
I believe the answer is:
asbestos
'dangerous material' is the definition.
(I've seen this before)
'a distress call about most' is the wordplay.
'distress call' becomes 'SOS' (an SOS message sent in an emergency).
'about' indicates putting letters inside.
'most' becomes 'best'.
'sos' placed around 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Fire-resistant material, a danger to health" , "we now know to be dangerous" , "Fire-retardant material" , "Firewall material" , "fire protection" .)