Advantage is seized by partners on the side (8)
I believe the answer is:
edgewise
'on the side' is the definition.
'advantage is seized by partners' is the wordplay.
'advantage' becomes 'edge' (edge is a kind of advantage).
'seized by' means one lot of letters goes inside another.
'partners' becomes 'we' (West and East are partners in the game bridge).
'is' put inside 'we' is 'wise'.
'edge'+'wise'='EDGEWISE'
(Other definitions for edgewise that I've seen before include "Barely, as hardly getting a word in, say" , "Sideways" .)