Clay men have some poison resistance (estimated at 50%), and have about half the hit points of a Wood Woad or Metal Construct. They also have hands allowing them to use spells like Burning Hands or Sickening Touch, but since they are always in combat, recasting the spells on them is impossible.
As with (almost) all minions in the Necromancer Animation skill tree, you can only have one summoned at a time, and they will drain a small amount of mana while they are active. If you run out of mana, the clay man will disappear. Similarly, they can only be given the control orders: 'tank' or 'protect'.