Change urban limits concerning an unknown number (6)

I believe the answer is:

rezone

'change urban limits' is the definition.

'concerning an unknown number' is the wordplay.
'concerning' becomes 're'.
'an unknown' becomes 'z' (algebra).
'number' becomes 'one' (one is a kind of number).
're'+'z'+'one'='REZONE'

I've seen this clue in The Independent.

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