Winning team is positive (6)
I believe the answer is:
upside
'positive' is the definition.
(an upside is a positive aspect of something)
'winning team' is the wordplay.
'winning' becomes 'up' (eg being up at half time in football).
'team' becomes 'side' (both can mean a sports team).
'up'+'side'='UPSIDE'
'is' is the link.
(Other definitions for upside that I've seen before include "Favourable aspect" , "advantage" , "Top surface of something" , "Positive aspect of a situation" .)