Diagnose disorder in US city (3,5)
I believe the answer is:
san diego
'us city' is the definition.
(San Diego is a city in California)
'diagnose disorder' is the wordplay.
'disorder' indicates anagramming the letters (a disordered version of the letters).
'diagnose' anagrammed gives 'SAN DIEGO'.
'in' is the link.
(Other definitions for san diego that I've seen before include "Californian port" , "Calif. naval base" , "Southern California naval base" , "Agonised over the Californian city" , "California naval port" .)