A cleric was a member of the leadership of a religion, such as a priest or preacher.
Clerics were present on Borath. Worf visited them after the destruction of the Enterprise in 2371. (DS9: "The Way of the Warrior", "The Sword of Kahless")
The rank of High Cleric also existed on Boreth, and the term cleric might have been used interchangeably with it. This is particularly the case in The Sword of Kahless, where Kor referenced the plot in "Rightful Heir" by saying ambitious clerics had cloned Kahless.
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")
Monean clerics believed that The Waters was a divine gift from the creators. (VOY: "Thirty Days")