A sign of diminution (5)

I believe the answer is:

minus

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'a sign' is the definition.
(minus sign is a kind of sign)

I cannot understand how the rest of the clue works.

(Other definitions for minus that I've seen before include "Deprived of" , "Lacking; negative" , "Lacking; below zero" , "Negative sign - negative thing" , "Take-away (sign)" .)

I've seen this clue in the Evening Standard.

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