It looks like it may have been attacked by spider mite. These mites are tiny and quite hard to see. They suck the life out of plant leaves, and clematis, unfortunately, are prone to them. Spray your plant with water and any webbing the mites have produced will show up more clearly. I found the info below on the web that, it may be of some help.

Pinch or prune off infested leaves or other plant parts. Wash plants with a strong stream of water to reduce pest numbers. Insects, such as ladybugs, lacewing, and predatory mites are important natural enemies. For best results, introduce them to your plant when pest levels are low to medium. If the mite population is high, use a least-toxic, short-lived natural pesticide to establish control, then release predatory insects to maintain control. Insecticidal soap or botanical insecticides can be used to spot treat heavily infested areas. Horticultural oils should be applied early in the season or late in the fall to destroy overwintering eggs.

Tip: Control strategies must take into account the fast development time of this pest, especially during warm weather when eggs are laid continuously. Just targeting the adults will do little good - repeat treatments are almost always necessary.

or it got frosted when you werent looking ....

before you attack .. perhaps you want to wait a bit, until a new set of leaves comes out ... if no damage.. frost... if they do the same.. get to work on the bugs ...

ken