permission vs clearance

permission and clearance both are nouns.

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

As nouns, clearance is a hyponym of permission; that is, clearance is a word with a more specific, narrower meaning than permission:

Other hyponyms of permission include conge, congee, allowance, dispensation, toleration.

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