Neither obtuse nor grave (and not right) (5)

I believe the answer is:

acute

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'right' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

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

Can you help me to learn more?

(Other definitions for acute that I've seen before include "Shrewd; serious" , "not seen in cafe here!" , "Penetrating; accent" , "Urgently pressing" , "Discerning" .)

I've seen this clue in The Guardian.

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