Spoiler alert, Claviarm is not actually a white mage. He just uses their pattern because it signifies being a healer. He's actually Dark element, though he will be the first to tell you that dark is not evil.
In brief, his abilities are restoration and protection, broadly interpreted, with a dark-elemental flavor. A few specific examples:
Healing: As the night allows us to rest and recover, so can darkness be used to heal--it is essentially a rapid regeneration, an acceleration and enhancement of natural recovery. And of course dark energy is capable of directly healing the undead, a feat which many healers can't perform.
Stealth Enhancement: As countless creatures pass unnoticed in the night, so does he who is one with the dark. Quieter and indescribably less visible, an observer may simply fail to notice the subject of this magic. What better protection is there than to never be perceived by those who would do you harm?
Barrier/Shield: A darkness barrier is extremely effective against magical or energy attacks--the incoming energy just vanishes into the darkness. But physical attacks are only partially impeded, as though striking through water.
Dispel: The strike of midnight marks the boundary between days, offering a fresh start, a clean slate. So it is that darkness is suited to ending any ongoing magical effects. Just as an incoming energy attack can be absorbed into a dark shield, darkness can also be employed to absorb the energy sustaining enchantments. And conceptually, dispelling is just another form of restoration, putting things back to the way they ought to be.
Unlike the white mages whose colors he wears, he has no ability with offensive enhancements or any sort of direct harm, nor any specific anti-undead capability. And while classic white mages are fully focused on their magic to the exclusion of any physical ability, Claviarm is more balanced--something like 65% magic, 35% physical.