Side having the advantage (4)
I believe the answer is:
edge
This clue is a double definition.
'side' is the first definition.
(I know that edge is a type of side)
'the advantage' is the second definition.
(having the edge is having an advantage)
'having' is the link.
(Other definitions for edge that I've seen before include "Border or cutting side of knife" , "Excitement; boundary" , "Brim" , "Keenness" , "Side, brink" .)