Possible Clues:

• Horseman perhaps always seizes opening
• Trained horse, regularly nervy, to submit
• Regular terminus -- no less -- for Badminton Horse Trials participant
• Horse is less jerky over time
• First woman almost to register as competitive rider
• Sportsperson riding eleven terrifying tackles
• Horse always found around hospital department
• Level ground, with no rain, for Badminton competitor?
• Horse in flat race, not Ascot's first
• Competing horse or rider