To be sharp, one needs to be clever (5)
I believe the answer is:
acute
'to be sharp' is the definition.
('acute' can be a synonym of 'sharp')
'one needs to be clever' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'needs' says to put letters next to each other.
'to be clever' becomes 'cute' (I've seen this in another clue).
'a'+'cute'='ACUTE'
(Other definitions for acute that I've seen before include "Short and severe" , "Grave" , "Shrewd; severe" , "Keen; severe" , "Extremely sharp or intense" .)