I hate to say so but it sure looks like a virus. Clematis are prone to several viruses, usually vectored by aphids or other sucking insects. The above photos look like the start of cucumber mosaic virus. And like with viruses in humans, there is nothing you can do for it. Generally, the recommendations are to rogue out and destroy the affected plant. It won't necessarily kill the plant but could cause it to weaken and even produce distorted flowers. The biggest risk is that it can spread to other clems in your garden or to other plants - it can affect a whole variety of different ornamentals as well as many edible crops.

I would take a copy of the photo and a sample of the vine into your local extension office for a concrete diagnosis before doing anything hasty, however :-))

Well I think I'm going to leave the two vines I have in this trellis fir now, they are blooming nicely. This fall I will dig them out...I'm sure I won't win a garden award for that choice

This is one of my many clematis.

These are photos of my clematis were submitted to clematis specialist and they seem to think it was the mosaic virus.

Whatever it is, it doesn't look right. Time to get a new clematis.