Margin of advantage (4)
I believe the answer is:
edge
I believe this clue is a double definition.
'margin' is the first definition.
(synonyms)
'advantage' is the second definition.
(having the edge is having an advantage)
'of' is the link.
(Other definitions for edge that I've seen before include "Surface boundary" , "small advantage" , "on which one is ill at ease?" , "Perimeter" , "Rim, lip" .)