Clue: Amateurish player stuffing ex-Wimbledon champion Serena, initially brazen (9)
Answer: SHAMELESS
Explanation: Definition: brazen
I think this is:
 HAM="Amateurish player (actor)", inside Monica SELES="ex-Wimbledon champion", plus initial of S-erena 
some research suggests that Monica Seles lost in a Wimbledon final, and did not become champion