Related Clues:

• Start of a bedtime story
• "___ Upon a Time in America"
• 'The Raven' opening
• ___ a day
• It's often enough
• Second word in many limericks
• "___ upon a time ..."
• ___ around the bases
• With 1-Down, every seven days
• "... __ and for all!"
• Start of an epic spaghetti western
• At some past point
• '... bitten, twice shy'
• On just one occasion
• On a single occasion only