Thank you for the photo and that certainly is powdery mildew on your clematis. Some varieties are more susceptible to this fungus than others but warm, humid weather and poor air circulation from crowded conditions are the most likely cause. The cure is to use a fungicide spray in the spring before real growth begins and then reapply as the label recommends. There are many products you could use but Funginex is probably the easiest to find. Be sure to clean up all around the plant and remove any dead leaves as this fungus overwinters on old foliage and nearby soil. The spores are also spread by the wind so you will need to pay close attention and treat the clematis again as soon as you see signs of the white powder. You did the right thing in cutting back your infected plant. I do not think you need to divide it as that could weaken the clematis and it sounds like the vine is just a few years old. A healthy clematis is more disease resistant to don't let your clematis get thirsty this summer and use an all purpose plant food such as Osmocote Plus to make sure all nutritional needs are met. The third year is a charm so I am expecting beautiful blooms form your clematis this summer. Keep growing, Marianne Binetti