Doctor to a sporting champion (11)
I believe the answer is:
protagonist
'champion' is the definition.
('protagonist' can be a synonym of 'champion')
'doctor to a sporting' is the wordplay.
'doctor' indicates an anagram.
'to'+'a'+'sporting'='toasporting'
'toasporting' anagrammed gives 'PROTAGONIST'.
(Other definitions for protagonist that I've seen before include "Key character in a novel" , "Leading player" , "Lead character" , "Chief character in a story" , "Advocate" .)