Representative from US got rare upset (9)
I believe the answer is:
surrogate
'representative' is the definition.
(I know that surrogate is a type of agent)
'us got rare upset' is the wordplay.
'upset' indicates an anagram.
'us'+'got'+'rare'='usgotrare'
'usgotrare' with letters rearranged gives 'SURROGATE'.
'from' acts as a link.
(Other definitions for surrogate that I've seen before include "(A) substitute in office" , "Argues about rot with substitute or deputy" , "Deputising substitute" , "One standing in" , "stand-in" .)