Possible Clues:

• Tough (anag.)
• Be obliged (to)
• Perhaps the South should have looked for this
• "You __ to know!"
• Moral obligation
• Anything purchased, except a book
• Crestfallen criminal would be obliged
• After introductory cancellation, did purchase have some duty?
• Has a duty (to do)
• Have a duty (to do)