Related Clues:

• "Knock, knock" response
• "Come in" command
• Carriage return key, nowadays
• Sign up to compete
• Go in and put in the ledger
• Come in or record
• Come or go in
• Enroll, sign up
• Go or come in
• Enlist, join up
• What the fish might do to get in like this under it for 18 down
• Do not go away before it is trouble inside
• Saw what the carp might be doing with this
• Where astronauts might get a drink - of a certain type
• One thus won't go and prise such an undertaking