Possible Clues:

• Part of the architecture of artificial intelligence
• Allow nothing to turn up over the door
• Allow nothing to turn up in the doorway
• Horizontal piece of timber over door
• In turn allow nothing to get over the door
• Beam that's essentially normal in telecommunications
• This might tell, perhaps, how to allow nothing to come back over the door
• In about to tell at the door
• Doorway topper
• Horizontal piece above a door