Diagnose ill in US city (3,5)
I believe the answer is:
san diego
'us city' is the definition.
(I've seen this before)
'diagnose ill' is the wordplay.
'ill' is an anagram indicator.
'diagnose' is an anagram of 'SAN DIEGO'.
'in' is the link.
(Other definitions for san diego that I've seen before include "City of S. California" , "Southern California naval base" , "Californian port" , "US city" , "Pacific port" .)