It sounds dry - it's the most common issue with clematis, especially if they're too near fences or walls, have a lot of other planting round them competing for moisture, or are in containers. They need a lot of water to keep them going, especially once they're mature and have full foliage on them.
It's also been very dry for long periods in many areas - even here it's been drier than usual through late spring into summer. I never have to water clems in the ground during spring and summer - but I've had to water some of mine this year.
Wilt is rare, and starts at the top of the plant, so if you still have green growth at the top, it isn't wilt.