Opposed to information of defensive substance (7)
I believe the answer is:
antigen
'defensive substance' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'opposed to information' is the wordplay.
'opposed to' becomes 'anti' (I've seen this before).
'information' becomes 'gen' (gen can informally mean information).
'anti'+'gen'='ANTIGEN'
'of' is the link.
Can you help me to learn more?
(Other definitions for antigen that I've seen before include "It stimulates immune responses - tag nine (anag)" , "Something inducing an immune response, neat gin (anag.)" , "toxin" , "Biological substance" , "Immunity-booster" .)