Argentina -- upset without a right means of defence (7)
I believe the answer is:
antigen
'means of defence' is the definition.
(antigens allow the body to produce antibodies)
'argentina upset without a right' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for antigen that I've seen before include "Something inducing an immune response, neat gin (anag.)" , "Biological substance" , "Substance inducing an immune response" , "It stimulates immune responses - tag nine (anag)" , "toxin" .)