Clematis wilt is a very specific problem - shoots suddenly wilt from the top downwards - only one shoot may be affected, or more. More generalised wilting is usually not enough water at the roots, but clematis also like their roots shaded, so if your pot is standing in full sun, the roots have probably got very hot recently.

You can grow the smaller clematis for a good five years in a pot at least 18 inches deep by 12 inches wide - I always insulate clematis containers with that thin, packing stuff that feels a bit like soft polystyrene, dunno what it's called. By smaller, I mean ones like The President and Bees Jubilee, those which get about 8-10 feet.

Alternatively, as clematis dies back in winter anyway, and they won't last forever in pots, how about trying summer climbers, such as Tropaleum peregrinum or Eccremocarpus scaber - they grow rapidly and flower all summer, you just need to start them from seeds in spring.

I would look for something that has interest all year round preferably an evergreen. If it is a sheltered, sunny spot then trachelospermum jasminoides would be good in a pot. You really need to say what aspect and part of the country as it will make a difference between hardy and tender subjects.