The advantage of keenness (4)
I believe the answer is:
edge
I believe this clue is a double definition.
'the advantage' is the first definition.
(having the edge is having an advantage)
'keenness' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for edge that I've seen before include "Rim or sharp side of knife" , "Create borders" , "Creep" , "marginal advantage" , "Steal" .)