Hat that’s key for Oscar in city (7)
I believe the answer is:
hamburg
'city' is the definition.
(Hamburg is an example)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(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" .)