Possible Clues:

• Amount carried puts poor horse in strain
• Ordered a gent to stand outside on duty
• All our ships can carry horse through sound
• Character concealing horse's weight
• The 21 across of the horse, or the ship - in the sound of it
• No agent (anag)
• Measured weight
• Weight of a sort
• Weight; ship's capacity
• Carrying capacity of horse in normal muscle condition