Detective encountering nonsense about one US city (7)
I believe the answer is:
detroit
'one us city' is the definition.
(I've seen this before)
'detective encountering nonsense about' is the wordplay.
'detective' becomes 'det' (abbreviation for detective).
'encountering' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'about' indicates putting letters inside.
'i' placed inside 'rot' is 'roit'.
'det'+'roit'='DETROIT'
Can you help me to learn more?
(Other definitions for detroit that I've seen before include "Motown, Mich." , "Michigan car city" , "City also called Mo'town in USA" , "city by a Great Lake" , "Major Great Lake port" .)