The Collaborative International Dictionary of English v.0.48:

Clearness \Clear"ness\, n.
   The quality or state of being clear.

   Syn: Clearness, Perspicuity.

   Usage: 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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

clearness
    n 1: free from obscurity and easy to understand; the
         comprehensibility of clear expression [syn: clarity,
         lucidity, lucidness, pellucidity, clearness,
         limpidity] [ant: abstruseness, obscureness,
         obscurity, reconditeness, unclearness]
    2: the quality of clear water; "when she awoke the clarity was
       back in her eyes" [syn: clearness, clarity,
       uncloudedness] [ant: opacity, opaqueness]