Possible Clues:

• Back gate
• Author of Pantomime at the post office (the back door)
• Sign attached to back of iron gate
• Notice number on that back door
• Entrance of officer to appropriate part of ship
• What gets stuck up on the northern gate
• Bill's name exhibited in gate
• The back gate of the river is at the back of the boat
• Bill covering new door
• Is the river so forbidding at the gate back there?