In sum, just a dash (5)

I believe the answer is:

minus

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'just a dash' is the definition.
I can't tell whether this definition defines the answer.

'in sum' is the wordplay.
'in sum' can be anagrammed to 'MINUS'.
Though, I'm unsure how the anagram is indicated.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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