Cross - judge defending old queen exempt from tax (4-4)

I believe the answer is:

zero-rate

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'exempt from tax' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'cross judge defending old queen' is the wordplay.
'cross' becomes 'zo' (cross between a yak and a cow).
'judge' becomes 'rate' (rating is a kind of judging).
'defending' indicates putting letters inside (inserted letters are defended).
'old queen' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'zo'+'rate'='zorate'
'zorate' placed around 'er' is 'ZERO-RATE'.

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I've seen this clue in The Independent.

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