Possible Clues:

• Nullify, as a 41-Across
• Perhaps to be too much on horseback makes one disregard all opposition (8)
• To be more important than being on tour?
• Rescind
• Annul
• Set aside blunder in Roman poet's note
• Annul sin burdening Roman writer close to Horace
• Function-canceling setting
• Cancel out boob in remake of video
• Reject out of hand request to transmit message on horseback