Possible Clues:

• Unpredictable factor
• Deuce, maybe
• Joker isn't a bit civilised joining Board
• Joker
• Player invited to compete, though not formally qualifying
• Fifth ace, perhaps, from Wimbledon outsider
• Right to enter competition without qualifying
• Dark horse, unbroken and eccentric
• Unqualified competitor given discretionary entry (often the joker!)
• Joker who gets in without qualifications