Leading team’s positive (6)
I believe the answer is:
upside
'positive' is the definition.
(an upside is a positive aspect of something)
'leading team's' is the wordplay.
'leading' becomes 'up' (up can mean winning a sports match).
'team' becomes 'side' (both can mean a sports team).
'up'+'side'='UPSIDE'
(Other definitions for upside that I've seen before include "Top surface of something" , "Favourable aspect" , "advantage" , "Positive aspect of a situation" .)