Possible Clues:

• 'There ___ to be a law!'
• ''You __ to know better''
• Indicator of probability searched for, initially lost
• Should do anything we’re told
• Unnamed duck could be anything at all
• Has a duty
• Cycling hard? This suggests duty or obligation, perhaps
• Has obligation, being hunted to hide head
• Is obligated
• "This ___ to do the trick"