plainness vs clearness

plainness and clearness both are nouns.

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

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

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

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

plainness (noun)	clearness (noun)
the appearance of being plain and unpretentious	free from obscurity and easy to understand; the comprehensibility of clear expression
an appearance that is not attractive or beautiful	the quality of clear water
clarity as a consequence of being perspicuous
the state of being unmixed with other material