She defeated Umar to become the Sorceress Supreme of the Dark Dimension. She basically now has Dormammu's role. She knows the same mystical arts as Dr. Strange (he taught her), but her Faltine bloodline makes it more natural for her.
What were the circumstances in their fight? I read that from her bio as well, but it isnt saying much. What are Umars feats? When Clea fought her, she was depowered.
The only reason that Strange defeats Umar and Dormammu is because of his intellect. Power has nothing to do with it. Umar and Dormammu will almost always have more power than Strange because they are high level magical beings, not just magic users like Strange. It's the same with Clea. I'm not sure if it's enough of an advantage for someone like Zatanna, but it's still a huge plus.