Chemical toxin affected attorney inside (7)
I believe the answer is:
oxidant
'chemical' is the definition.
(I have seen 'Harmful chemical' mean 'oxidant' so perhaps 'chemical' could also mean 'oxidant')
'toxin affected attorney inside' is the wordplay.
'affected' indicates an anagram.
'attorney' becomes 'DA' (abbreviation for District Attorney).
'inside' means one lot of letters goes inside another.
'toxin' with letters rearranged gives 'oxint'.
'oxint' placed around 'da' is 'OXIDANT'.
(Other definitions for oxidant that I've seen before include "Harmful chemical" , "agent causing deterioration" , "Producer of reddish coat" , "Something creating rust" , "it produces rust" .)