Capital that was once part of Denmark Crossword Clue

 The answer to the crossword clue "Capital that was once part of Denmark" is Copenhagen.

Copenhagen is the capital of Denmark. It was once part of the Kalmar Union, which was a union of Denmark, Norway, and Sweden. The Kalmar Union lasted from 1397 to 1523.

Here are some other possible answers to the clue:

Copenhagen

Stockholm

Oslo

Helsinki

Reykjavík

Tallinn

The specific answer that is correct will depend on the context of the crossword puzzle.

I hope this helps! Let me know if you have other crossword clues for me.