Clearness, n.
The quality or state of being clear.
Syn. -- Clearness, Perspicuity. Clearness has reference to our ideas, and springs from a distinct conception of the subject under consideration. Perspicuity has reference to the mode of expressing our ideas and belongs essentially to style. Hence we speak of a writer as having clear ideas, a clear arrangement, and perspicuous phraseology. We do at times speak of a person's having great clearness of style; but in such cases we are usually thinking of the clearness of his ideas as manifested in language. "Whenever men think clearly, and are thoroughly interested, they express themselves with perspicuity and force." Robertson.
Shades of meaning
Fernald lists clearness only as an antonym, under insanity.
He lists it without comment; the entries above explain its neighbors.