Principal Zulu city (5)

I believe the answer is:

mainz

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'city' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?

'principal zulu' is the wordplay.
'principal' becomes 'main' (both can mean major).
'zulu' becomes 'Z' (phonetic alphabet: alpha, bravo, charlie etc.).
'main'+'z'='MAINZ'

Can you help me to learn more?

(Another definition for mainz that I've seen is " port in Germany".)

I've seen this clue in The Times.

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