Negative mark appended to wine to make point (4)

I believe the answer is:

crux

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'point' is the definition.
(I know that crux is a type of point)

I cannot understand how the rest of the clue works.

(Other definitions for crux that I've seen before include "The most important thing" , "Decisive, baffling, point" , "Key point" , "Core" , "Pivotal point - cross" .)

I've seen this clue in The Independent.

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