An advantage to the side (4)
I believe the answer is:
edge
This clue is a double definition.
'an advantage' is the first definition.
(having the edge is having an advantage)
'the side' is the second definition.
(edge is a kind of side)
'to' is the link.
(Other definitions for edge that I've seen before include "small advantage" , "Border or cutting side of knife" , "on which one is ill at ease?" , "Keenness" , "Final line" .)