A sign of insufficiency? (4)

I believe the answer is:

plus

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

I can't explain the remainder of the clue.

(Other definitions for plus that I've seen before include "Advantage; also" , "Positive and not a negative" , "being a little cross" , "An advantage; positive" , "To the French it's more" .)

I've seen this clue in the Evening Standard.

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