It means less in maths (5)

I believe the answer is:

minus

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'less in maths' is the definition.
'minus' can be an answer for 'less' (I've seen this before). I'm unsure of the 'in maths' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for minus that I've seen before include "'Take away' sign" , "Take-away (sign)" , "Less, without" , "Lacking; below zero" , "Deprived of" .)

I've seen this clue in the Evening Standard.

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