Only you can know if this clematis is sitting in a drafty spot, it doesn't look like virus to me and I wonder if you are keeping it well enough watered. Planted in the corner as it is it could be in a rain shadow, which way does it face, south, east or west?

It faces south but it's beside a pillar rather than in a big corner. It probably has been too dry but as the others along the wall are okay I just wanted to be sure about a virus. It's been so dry up here for weeks.

then you need to water Clematis hate to be dry!