A cleric was a member of the leadership of a religion, such as a priest or preacher.
The story of the Sword of Kahless was originally known only by the Klingon High Clerics, and was never written down in the sacred texts. The clerics of Boreth, lead by Koroth, cloned Kahless in 2369 using DNA from the Knife of Kirom in order to unite the Klingon people, and used the clone's knowledge of the story as evidence that he was in fact the true Kahless. (TNG: "Rightful Heir")
Dala and Mobar posed as clerics (as a Sister and Brother respectively) from Selnia Prime when they met Neelix and Tom Paris on the planet in 2376. (VOY: "Live Fast and Prosper")