Substitute from US got rare treatment (9)
I believe the answer is:
surrogate
'substitute' is the definition.
(surrogate is a kind of substitute)
'us got rare treatment' is the wordplay.
'treatment' is an anagram indicator.
'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" , "Deputising substitute" , "stand-in" , "Argues about rot with substitute or deputy" , "One standing in" .)