Horse rider always circling Tolkien creature

• Level ground, with no rain, for Badminton competitor?
• Competing horse or rider
• Leader in equestrianism, victory key for competitive horse
• Flat race without a horse
• Competitive rider always covers slit in jacket
• Equestrian competitor
• Sportsperson held hostage by eleven terrorists

Wiktionary Word definitions in Wiktionary
n. 1 a horse or type of horse used in the equestrian sport of eventing. 2 (context equestrian English) the rider of a horse used in eventing.