What is Clearance ?
Clearance is (noun) 1. the act of removing obstacles such as trees or old buildings from land The clearance of the slums from the town centre will make land available for building. The government has introduced a programme of slum clearance. 2. space for something to pass through The lorry can get through the entrance with about twenty centimetres clearance on either side. 3. permission to do something The control tower gave the plane clearance to land. (NOTE: no plural)
source: Easier English, Student Dictionary Upper Intermediate Level