A cry for help about worst poisonous substance (8)
I believe the answer is:
asbestos
'poisonous substance' is the definition.
'asbestos' can be an answer for 'substance' (asbestos is a kind of substance). I am unsure of the 'poisonous' bit.
'a cry for help about worst' is the wordplay.
'cry for help' becomes 'sos' (an SOS message sent in an emergency).
'about' indicates putting letters inside.
'worst' becomes 'best' (both can mean to beat someone).
'sos' placed around 'best' is 'sbestos'.
'a'+'sbestos'='ASBESTOS'
Can you help me to learn more?
(Other definitions for asbestos that I've seen before include "Heat-resistant material" , "Fireproof material, now known to be dangerous" , "fire protection" , "Fire-resistant material, a danger to health" , "Fire-retardant material" .)