Diagnose disorder in US city (3,5)
I believe the answer is:
'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' anagrammed gives 'SAN DIEGO'.
'in' acts as a 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" .)