Crossword Genius

Thirty minutes in old city? (4,2,4)

Ross

I believe the answer is:

half an hour

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

'thirty minutes' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'old city?' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is found within the answer.
'city?' could be 'ur' (ancient Sumerian city state) and 'ur' is found within the answer.
This explanation may well be incorrect...

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish Times.
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