Note about US city (4)
I believe the answer is:
oner
'note' is the definition.
(historical term for a one pound note)
'about us city' is the wordplay.
'about' says the letters should be written in reverse.
'us city' becomes 'Reno' (city in Nevada).
'reno' backwards is 'ONER'.
(Other definitions for oner that I've seen before include "Big blow" , "something unique" , "Out-of-date banknote" , "Unique person" , "Remarkable thing" .)