Sounds like it could be wilt.

Clematis plants can recover from wilt, because it does not attack their root system.
At the first sign of withering or drying, cut the affected stems back to ground level. It sounds severe, but it can save your plant. Dispose of the cuttings somewhere other than the compost. New shoots should emerge from the base shortly, although Washington State University Extension’s fact sheet said “One resource on this subject states that renewed shoots can appear up to three years after the problem, so do not lose heart if this happens.”

Keep the roots water, even if there is no top growth. Remove all remaining growth in the fall and dispose of it. The fungus can over-winter in the dead foliage.

If you wish to use a preventative fungicide, sulfur is recommended.