Girl entertains star in US city (3,5)
I believe the answer is:
las vegas
'us city' is the definition.
(I've seen this before)
'girl entertains star in' is the wordplay.
'girl' becomes 'lass' (lass is a kind of girl).
'entertains' indicates putting letters inside.
'star in' becomes 'vega' (I can't explain this - if you can you should give a lot more credence to this answer).
'lass' enclosing 'vega' is 'LAS VEGAS'.
Can you help me to learn more?
(Other definitions for las vegas that I've seen before include "entertainment centre" , "Nevadan city" , "Gambling centre in USA" , "US gambling city" , "US resort - with 23d" .)