Popular team getting runs, one with special advantage (7)
I believe the answer is:
insider
'one with special advantage' is the definition.
(as in insider trading)
'popular team getting runs' is the wordplay.
'popular' becomes 'in'.
'team' becomes 'side' (both can mean a sports team).
'getting' is a charade indicator (letters next to each other).
'runs' becomes 'r' (cricket abbreviation).
'in'+'side'+'r'='INSIDER'
(Other definitions for insider that I've seen before include "One privy to knowledge as an organisation member" , "One dealing illegally" , "Wrong sort of trading" , "Accepted member of group, in the know" , "person in the club" .)