Clear-sightedness is the name of a quality: the ability to understand a problem or situation well. It sits beside the adjective clear-sighted. You praise someone's clear-sightedness when they see what is really happening and are not fooled by hope, fear, or a pretty story. It is a rather formal, bookish word. In ordinary talk people are more likely to say she sees things clearly than I admire her clear-sightedness. Though actually, the extra -ness does not add a new meaning — it only turns the adjective into a thing you can name.