This clematis is covered in a very advanced case of powdery mildew fungus! This is about as bad of a case I have ever seen. This article is about powdery mildew on dogwoods and crapemyrtles but the information would apply to clematis.

Powdery mildew is a type of fungus that grows on the surfaces of plant leaves and flowers. It feeds on the plant and can make plants look terrible. It usually is not fatal.

Control

Fungicides can protect plants when applied before at the start of an infection. In this case, it is too late to spray.  It probably would be best to cut the vine back to the roots and let it regrow from the roots. Learn more here.