A sign of crossness? (4)

I believe the answer is:

plus

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

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

(Other definitions for plus that I've seen before include "Positive sign" , "Distant" , "And also; a bonus" , "The opposite of minus" , "An advantage; positive" .)

I've seen this clue in the Evening Standard.

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