Advantage gained by leading team (6)
I believe the answer is:
upside
'advantage' is the definition.
(both can mean a positive aspect of something)
'leading team' is the wordplay.
'leading' becomes 'up' (similar in meaning).
'team' becomes 'side' (both can mean a sports team).
'up'+'side'='UPSIDE'
'gained by' acts as a link.
(Other definitions for upside that I've seen before include "Favourable aspect" , "advantage" , "Top surface of something" , "Positive aspect of a situation" .)