Unaccompanied, but very much upset in this city (4)

I believe the answer is:

oslo

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'this city' is the definition.
(capital of Norway)

I don't understand the rest of the clue.

(Other definitions for oslo that I've seen before include "Norway's capital city" , "Go solo to European capital" , "Nordic capital" , "Capital in throes of go-slow" , "Norwegian chief port" .)

I've seen this clue in The Guardian.

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