Sharp or severe (5)
I believe the answer is:
acute
This clue is a double definition.
'sharp' is the first definition.
(similar in meaning)
'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" .)