Serious but not acute (5)

I believe the answer is:

grave

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This could be a double definition.

'serious' is the first definition.
(both can mean solemn)

'not acute' is the second definition.
'grave' can be an answer for 'acute' (thesaurus). I am not sure about the 'not' bit.

'but' acts as a link.

Can you help me to learn more?

(Other definitions for grave that I've seen before include "of importance" , "Grievous, serious" , "Sober" , "Solemn, serious" , "Last resting place" .)

I've seen this clue in The Globe and Mail.

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