Become established outside a US city (7)
I believe the answer is:
seattle
'us city' is the definition.
(city in Washington, USA)
'become established outside a' is the wordplay.
'become established' becomes 'settle' (synonyms).
'outside' indicates putting letters inside.
'settle' going around 'a' is 'SEATTLE'.
(Other definitions for seattle that I've seen before include "US West coast port" , "Sleepless in --, film" , "major US port" , "US Pacific seaport" , "Let's eat at this US city" .)