Opposed to information describing component of vaccine (7)
I believe the answer is:
antigen
'component of vaccine' is the definition.
(a vaccine may contain antigens)
'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'
'describing' acts as a link.
(Other definitions for antigen that I've seen before include "toxin" , "Immunity-booster" , "Substance inducing an immune response" , "It stimulates immune responses - tag nine (anag)" , "Something inducing an immune response, neat gin (anag.)" .)