Luckily I have never seen it on my own clematis but from your description it seems it can be clematis wilt. It is described in the FAQ section of this forum.

It could also be a broken vine....some critter eating on it...And as was said wilt..those that do that ..just prune to the ground....Jeanne

Unfortunately, they do this on a routine basis. Prune out the dead section and it should recover, they do tend to grow out of it. Eventually.