Crossword Genius

Mild union developed the old city (9)

Ross

I believe the answer is:

londinium

I'm a little stuck... Click here to teach me more about this clue!

'old city' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'mild union developed' is the wordplay.
'developed' is an anagram indicator.
'mild'+'union'='mildunion'
'mildunion' is an anagram of 'LONDINIUM'.

'the' is the link.

Can you help me to learn more?

(Another definition for londinium that I've seen is " Roman city on the Thames".)

I've seen this clue in the Sydney Morning Herald.
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