An extra to make you cross? (4)

I believe the answer is:

plus

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'an extra' is the definition.
(plus can mean more than expected)

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

(Other definitions for plus that I've seen before include "Coupled with" , "Positive sign" , "And, arithmetically" , "advantageous" , "being a little cross" .)

I've seen this clue in the Evening Standard.

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