Meaningless mathematical sign? (5)

I believe the answer is:

minus

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'sign?' is the definition.
(I know that minus is a type of sign)

I cannot understand how the rest of the clue works.

(Other definitions for minus that I've seen before include "Deprived of" , "Reduced by the subtraction of - thus?" , "Disadvantage" , "Lacking; negative" , "Negative sign - negative thing" .)

I've seen this clue in The Globe and Mail.

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