A number in agreement with ten (3)

I believe the answer is:

six

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'a number' is the definition.
(six is a kind of number)

'agreement with ten' is the wordplay.
'agreement with' becomes 'si' (I can't explain this - if you can you should believe this answer much more).
'ten' becomes 'X' (Roman numeral).
'si'+'x'='SIX'

'in' acts as a link.

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(Other definitions for six that I've seen before include "Knock for . . . - surprise or defeat" , "Maximum cricket hit" , "Half-dozen" , "Number of US moon landings" , "VI" .)

I've seen this clue in the Evening Standard.

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