Opposed to information revealing toxin (7)
I believe the answer is:
antigen
'toxin' is the definition.
(I've seen this before)
'opposed to information' is the wordplay.
'opposed to' becomes 'anti' (anti means opposing something).
'information' becomes 'gen' (gen can informally mean information).
'anti'+'gen'='ANTIGEN'
'revealing' acts as a link.
(Other definitions for antigen that I've seen before include "Biological substance" , "Substance inducing an immune response" , "toxin" , "It stimulates immune responses - tag nine (anag)" , "Something inducing an immune response, neat gin (anag.)" .)