Not an intelligent sort of angle? (6)

I believe the answer is:

obtuse

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'sort of angle?' is the definition.

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

(Other definitions for obtuse that I've seen before include "Slow-witted; type of angle" , "Mentally slow - kind of angle" , "Lacking in insight" , "Blunt, dull-witted" , "(Of angle) between 90deg and 180deg - dull" .)

I've seen this clue in The Sun and The Times.

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