Run, say, number on estate? (3)

I believe the answer is:

reg

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'number on estate?' is the definition.
'reg' can be an answer for 'number' (registration number on a vehicle). I'm unsure of the 'on estate?' bit.

'run say' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'say' becomes 'eg' (both can mean for example).
'r'+'eg'='REG'

Can you help me to learn more?

I've seen this clue in The Times.

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