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' is an anagram indicator (a disordered version of the letters).
'diagnose' is an anagram of '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" .)