Clerics generally appear near Holy Relics, or with highly destructive monsters. In the latter case, a group of 2-7 clerics continuously cast All Better on the monster. They never appear with undead creatures; if near a vampire or other undead, they will cast Turn Undead upon it.
However, clerics will not spawn on the same floor as any undead enemy. If, however, you steal a Holy Relic from a cleric, he will follow you horrendously; you can lead the cleric to a floor with undead, and let him do all the work.
Clerics will likewise follow any player with negative hit points ("zombie" status). There is a higher chance that they will spawn when the player has negative hit points (as the player isn't truly undead, this does not contradict the previous paragraph).