Opposite of acute, for an angle (6)
I believe the answer is:
obtuse
This clue is a double definition.
'opposite of acute' is the first definition.
('obtuse' is the opposite to 'acute')
'for an angle' is the second definition.
(obtuse angle is a kind of angle)
(Other definitions for obtuse that I've seen before include "To be thus blunt" , "Mentally slow - kind of angle" , "Angles could be" , "angle, perhaps" , "(Of angle) between 90deg and 180deg - dull" .)