I have a few thoughts on PvP and clerics. I play a cleric on Vallon Zek and
occationally battle other players. In my experience, clerics are not good
offensively, but rather good defensively, especially grouped.
Once a necromancer cast dots on me (engulfing darkness and boil blood I
believe), plus fear. I dropped HPs rapidly, but managed to cast a couple of
heals on myself, and then the DOTs wore off.

Can clerics generally survive a fight versus casters by healing themselves?
If I cast 4 greater heals on myself I can heal for 1200+ damage. Although I
am not sure, I doubt a wizard will have the mana to deliver 2000 damage at
my level (24), and some of his damage might be resisted. Assuming I have
full armour, I should be able to root and melee the caster to death.

Other useful spells are blindness and stun. I use the low level variants of
stun since they take less mana. Divine aura is useful since the attacker
doesn't always observe that I am invulnerable, especially casters, since
they don't get any damage information (="blabla hit by non melee for 140
damage" etc).

Buying an egg shaped pumice is also a good investment, although it is quite
expensive. It is a magic item sold by some vendors (in Neriak, the one near
the bank) and has 5 charges of "pillage enchantment", that will cancel
several effects like charm/DOTs (and buffs) at the same time.