Related Clues:

• 'Hop aboard!'
• Formal response to a knock
• Offer to a hitcher
• Door inscription
• Come out from the wings
• 'Come in,' if you're a super-dramatic movie villain
• '___ the Wu-Tang (36 Chambers)' (hip-hop album peppered with kung fu movie samples)
• Come into some barren terrain
• Computer-key word
• Carriage return equivalent, today
• Go in hospital department with expression of hesitation
• Going in to pen, terrified to go in!
• Register formally as a participant
• Begin upon
• Key hit when starting a new paragraph