Miss boxing star in US city (3,5)
I believe the answer is:
las vegas
'us city' is the definition.
(I've seen this before)
'miss boxing star in' is the wordplay.
'miss' becomes 'lass' (lass is a kind of miss).
'boxing' indicates putting letters inside ('box' can mean contain, as in to put in a box).
'star in' becomes 'vega' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lass' going around 'vega' is 'LAS VEGAS'.
Can you help me to learn more?
(Other definitions for las vegas that I've seen before include "Gambling centre in USA" , "Gambling city in Nevada, USA" , "US gambling city" , "Gambling capital of the world?" , "Famous gambling city in Nevada" .)