Looking forward to an answer

Some cultivars, mostly the large-flowered ones, do have a tendency to wilt. Clematis wilt is a fungal disease (Ascochyta clematidina) and causes the complete collapse of a plant. It can't be cured, but you can help the plant to send up new shoots by cutting back all dead stems and burning them, as well as any fallen leaves, which carry spores.

One reason for planting Clems deep is to help them to grow new shoots from the rootball in the following year.

Are you sure it's 'clematis wilt' and not just the plant dying back ?

Most clematis develop wilted brown/black leaves when they're too dry - their stems are often still alive.

It would be worth making sure that it's actually 'wilt' first - which as Spritz has explained is infact a fungal disease.

How old is the plant, is it kept very well watered, is its site right ...... these questions need to be asked before concluding it's wilt.