Clematis Wilt is a systemic condition that only really affects plants that are under sufferance. Healthy Clematis should not suffer from the wilt, however if you have a clematis that is of the large flowered kind ( eg Group 2 varieties ) that has all of a sudden drooped overnight ( or very quickly ) then water in a copper based fungicide e.g Doff "plant disease control" ( or similar )
Then cut the plant back to where the the wilt has started ( depending on how long it's been will depend on how far the wilt has travelled down the stem) then destroy the affected stems. Water regular and start to feed with a potash based feed ( tomato food ) as soon as fresh growth starts to appear.
As long as the clematis has been planted deep in the first instance, the plant will re-shoot even if the wilt has travelled all the way to ground level.
The RHS also have some more info and pics on the subject of 'Clematis Wilt' Click Here