A diminishing sign (5)

I believe the answer is:

minus

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

I don't understand the remainder of the clue.

(Other definitions for minus that I've seen before include "Negative arithmetic operation" , "Reduced by the subtraction of - thus?" , "(Of temperature) below zero" , "Less, without" , "Lacking; below zero" .)

I've seen this clue in the Evening Standard.

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