How old is the clematis? How long in the pot? What sort of pot - plastic, terracotta, glazed? How big is the pot/ the plant? What is the situation - against a wall/fence/direction etc?
Do you water the plant or just leave it to nature?
By "wilting" do you mean that the stems are soft droopy - probably just needs a good drink - or is it something more severe such as suddenly losing leaves?

Can you post photographs?

What variety of clematis is it? Clematis Wilt only affects hybrid varieties, not species, and usually starts with one or two stems suddenly wilting at the top, spreading downwards. If the Wilt affected stems are not removed, the whole plant will be affected. It is a disorder of Clematis.
Other causes of wilting are insufficient water, but usually the whole plant droops, doesn't start at the top, so if your clematis has been in the same pot for more than 2 years, its possible its rootbound and can't hang on to enough water.