Crossword Genius

Hat that’s key for Oscar in city (7)

Ross

I believe the answer is:

hamburg

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'city' is the definition.
(Hamburg is an example)

I don't understand the remainder of the clue.

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(Other definitions for hamburg that I've seen before include "Port near the mouth of the Elbe" , "German state and largest port" , "German city-seaport on the Elbe" , "A meat dish named after the city" , "North German city" .)

I've seen this clue in The Times.
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