Clearance

noun

1. The act of clearing or something (such as a space) cleared
2. The distance between two moving objects, especially between parts of a machine
3. The height or width of a tunnel, bridge or other passage, or the distance between a vehicle and the walls or roof of such passage; a gap, headroom.
4. A permission for a vehicle to proceed, or for a person to travel.

   "He got clearance to travel to America, even though he had previous links to terrorists"

5. A permission to have access to sensitive or secret documents or other information.