Harmful stuff when former footballer so drunk (8)
I believe the answer is:
asbestos
'harmful stuff' is the definition.
'when former footballer so drunk' is the wordplay.
'when' becomes 'as'.
'former footballer' becomes 'best' (George Best).
'drunk' indicates an anagram.
'so' is an anagram of 'os'.
'as'+'best'+'os'='ASBESTOS'
(Other definitions for asbestos that I've seen before include "Survivor of heat" , "Fire-resistant mineral" , "Insulating mineral" , "Heat-resistant fibrous mineral" , "Fire-retardant material" .)