British rebels rising up in Argentine city (5)
I believe the answer is:
iceni
'british rebels' is the definition.
(ancient Britons who rebelled against the Romans)
'rising up in argentine city' is the wordplay.
'rising up' says the letters should be written in reverse (in a down clue, letters go up).
'in' indicates a hidden word.
'ineci' is hidden in the letters of 'argentine city'.
'ineci' in reverse letter order is 'ICENI'.
(Other definitions for iceni that I've seen before include "Old tribe" , "Queen Boudicca's people" , "Ancient British tribe" , "Old people" , "Boudica's tribe" .)