Possible Clues:

• Racing rarity
• Horse-racing hat trick
• Sporting achievement that went to, say. Paul's head
• Prize for series of victories is a tiara
• Sir Barton was the first to win it
• American Pharoah's 2015 feat
• Headgear for a certain batting champ?
• Thoroughbred racing coup first achieved in 1919
• What can be won at rugby? Nonsense written about Lomu's first cap
• Slugger's achievement