The Clarius system was a star system which was located somewhere in the Alpha or Beta Quadrant. This system was home to arcybite mining refineries.
In 2369, a Ferengi named Nava completed a takeover of these refineries. (DS9: "The Nagus")
In the script for "The Nagus", Clarius was listed in the pronunciation guide, where the term was pronounced as "CLAIR-ee-us". [1]
Also see: the similarly named Clarus system near Ferenginar.