clearance vs permission

clearance and permission both are nouns.

Word	Noun	Adjective	Verb	Adverb
clearance	Yes	No	No	No
permission	Yes	No	No	No

As nouns, permission is a hypernym of clearance; that is, permission is a word with a broader meaning than clearance:

Other hypernyms of clearance include license, permit.

clearance (noun)	permission (noun)
permission to proceed	the act of giving a formal (usually written) authorization
the distance by which one thing clears another; the space between them	approval to do something
vertical space available to allow easy passage under something