Possible Clues:

• Word Johnson misdefined, making the old man severe
• It's between the fetlock and hoof
• Dad's bird is on the hoof
• Is the bird of Father's on his horse?
• The old man? Strict, bit of a nag
• 'The knee of an horse' (Johnson)
• Pressure at the rear part of horse
• Father authoritarian, bit of a nag
• Wrongly-defined part of horse, dad — tough!
• Daddy's bird is on the hoof