It means less (5)

I believe the answer is:

minus

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'less' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for minus that I've seen before include "(Of temperature) below zero" , "Less, without" , "Reduced by the subtraction of - thus?" , "Deprived of" , "Take-away (sign)" .)

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