Not a clever sort of angle (6)

I believe the answer is:

obtuse

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'sort of angle' is the definition.

I cannot understand how the remainder of the clue works.

(Other definitions for obtuse that I've seen before include "Blunt - stupid - insensitive" , "Blunt, dull-witted" , "(Of angle) between 90deg and 180deg - dull" , "Insensitive - more than a right angle" , "not acute?" .)

I've seen this clue in the Evening Standard.

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