Sharp or severe (5)
I believe the answer is:
acute
This clue is a double definition.
'sharp' is the first definition.
(I know that sharp can be written as acute)
'severe' is the second definition.
(synonyms)
(Other definitions for acute that I've seen before include "Sharp-pointed - keenly perceptive" , "Incisive" , "Sharp, angle or mind" , "quick on the uptake" , "Sharp; type of accent" .)