Sharpness is an advantage (4)
I believe the answer is:
edge
This clue is a double definition.
'sharpness' is the first definition.
('edge' can be a synonym of 'sharpness')
'an advantage' is the second definition.
(edge is a kind of advantage)
'is' acts as a link.
(Other definitions for edge that I've seen before include "Hem" , "Boundary; advantage" , "Force" , "Some bordering" , "Surface boundary" .)