Related Clues:

• Security requirement
• It's heard before a liftoff
• 'A Chorus Line' number
• See 34-Down
• Formula ___
• 'Blastoff!' preceder
• It's inseparable
• ''--- True Thing'' (1998)
• Better than none
• Stop at the door, perhaps
• The loneliest number, in song
• Starting square
• Joker
• Single thing
• Funny guy