perspicuity vs clearness

perspicuity and clearness both are nouns.

Word	Noun	Adjective	Verb	Adverb
perspicuity	Yes	No	No	No
clearness	Yes	No	No	No

As nouns, clearness is a hypernym of perspicuity; that is, clearness is a word with a broader meaning than perspicuity:

perspicuity: clarity as a consequence of being perspicuous
clearness: free from obscurity and easy to understand; the comprehensibility of clear expression

Other hypernyms of perspicuity include clarity, limpidity, lucidity, lucidness, pellucidity.

perspicuity (noun)	clearness (noun)
clarity as a consequence of being perspicuous	free from obscurity and easy to understand; the comprehensibility of clear expression
	the quality of clear water