Champion rugby player playing in French team initially (9)
I believe the answer is:
proponent
'champion' is the definition.
(both can mean an endorser or supporter)
'rugby player playing in french team initially' is the wordplay.
'rugby player' becomes 'prop' (type of rugby player).
'playing' becomes 'on' (on can mean playing or performing).
'in french' becomes 'en' (French word for 'in').
'initially' says to take the initial letters.
The initial letter of 'team' is 't'.
'prop'+'on'+'en'+'t'='PROPONENT'
(Another definition for proponent that I've seen is " Advocate (of a theory, etc)".)