Opposed to understanding vaccine (7)
I believe the answer is:
antigen
'understanding vaccine' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'opposed to' is the wordplay.
'opposed' becomes 'anti' (similar in meaning).
'to' becomes 'gen' (I am not sure about this - if you are sure you should believe this answer much more).
'anti'+'gen'='ANTIGEN'
Can you help me to learn more?
(Other definitions for antigen that I've seen before include "It stimulates immune responses - tag nine (anag)" , "Immunity-booster" , "Something inducing an immune response, neat gin (anag.)" , "toxin" , "Substance inducing an immune response" .)