Not obtuse (5)
I believe the answer is:
acute
'not obtuse' is the definition.
('acute' is the opposite to 'obtuse')
This is the entire clue.
(Other definitions for acute that I've seen before include "Excruciating" , "Sharp or crucial" , "quick on the uptake" , "Critical: shrewd" , "Very perceptive, having sharp mind" .)