A couple of notes about a US city (5)
I believe the answer is:
miami
'us city' is the definition.
(I've seen this before)
'a couple of notes about a' is the wordplay.
'a couple of' shows that letters should be duplicated.
'notes' becomes 'mi' ().
'about' indicates putting letters inside.
'mi' duplicated is 'mimi'.
'mimi' going around 'a' is 'MIAMI'.
(Other definitions for miami that I've seen before include "tourist centre" , "US town" , "I put out of action when returning" , "state" , "place associated with vice?" .)