Sharp, severe (5)
I believe the answer is:
acute
I believe 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 "quick on the uptake" , "Severe; French accent" , "Incisive" , "Canny" , "Short and severe" .)