Sign of augmentation (4)

I believe the answer is:

plus

I believe this clue is a double definition.

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

'augmentation' is the second definition.
(thesaurus)

'of' is the link.

(Other definitions for plus that I've seen before include "And, arithmetically" , "mark for arithmetic" , "being a little cross" , "And also; a bonus" , "credit" .)

I've seen this clue in the Evening Standard.

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