Please check and make sure you are not confusing "wilt" with a broken stem

It is not a broken stem.
The same stem has green leaves in health.

Perhaps a lack of water, after all, is a plant that lives in China int the forests between 400-2000 m.

Species clematis are seldom ever bothered by 'wilt'. C. urophylla can die off from phytophthora if overwatered or the soil excessively moisture retentive - phytophthora is a quite different problem (a root rot, so dieback is from the bottom up, rather than wilt, which is from the top down) but can have a similar appearance.

In general, evergreen clematis tend to be less water demanding than deciduous ones.