"Clearness" is an unusual word. "Clarity" is better.
Student or Learner
The title says it: is "clearness" a word? Does it work as a synonym of "clarity"?
The book lacks some clarity/clearness.
Clearness is usually used to indicate opacity, translucence or lucidity (more mental).
clearness of the eyeglass lens
clearness of the window pane
clearness of the mind (his mind is clear of impure thoughts)
Clearness can be synonymous with clarity:
The abstract he wrote for his thesis was clear. The clearness of the abstract was apparent. He wrote it with great clarity.
Clarity is more associated with the written word or the spoken word than clearness (clarity of speech, clarity of writing).