Learner's definition of CLEARANCE
1 a : an official decision saying that someone has permission to do something
You'll have to get clearance [=approval] from management to go ahead with the project.
The documents are only available to people with security clearance [=special permission given only to people who are approved to know or see secret things] from the government.
[count] b : official permission for an aircraft, pilot, etc., to do something
2 : the amount of space between two things
3 [noncount] : the act or process of removing things (such as trees or old buildings) from an area