Clerics don't appear as random enemies, and don't use any Spells in combat. They may as well just be another Merchant. In fact the only time you'll have to fight a Cleric is in the era of Legends. They don't drop anything useful, either, making them a surprisingly uninteresting Creature.
There are no aggressive Clerics in the game, though you'll have to fight the one in Castle Fire if you want to reach the cells. They aren't worth a ton of experience, but also don't have any special attacks like you might expect.