In mathematics, it refers to a number (4)

I believe the answer is:

them

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'refers to a number' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'in mathematics it' is the wordplay.
'in' says the answer is hidden in the clue.
'THEM' is hidden in the letters of 'mathematics it'.

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(Other definitions for them that I've seen before include "Certainly not us, the other lot" , "Let -- eat cake" , "Others" , "(To/from) those people" , "Those people (as object)" .)

I've seen this clue in the Evening Standard.

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