Possible Clues:

• Dominant across old part of Yorkshire
• Like a jockey who's fed up?
• More important than any other considerations
• Dominant, ruling part of Yorkshire once
• Principal getting horse knackered?
• Finished up using one's authority to cancel
• More important than (any other considerations)
• Deliveries part of Yorkshire once found most important
• Most important maiden possibly on horseback
• Main reason for horse being tired?