Champion ahead in game (7)
I believe the answer is:
support
'champion' is the definition.
(champion can mean to support or promote)
'ahead in game' is the wordplay.
'ahead' becomes 'up' (similar in meaning).
'in' means one lot of letters goes inside another.
'game' becomes 'sport' (I've seen this before).
'up' inserted inside 'sport' is 'SUPPORT'.
(Other definitions for support that I've seen before include "base" , "Encourage - endorse" , "Bear the weight of (something)" , "Stay" , "Provide financial backing or bolster" .)