It could be a couple of things but my first suspicion would be that you somehow damaged one of the stems coming from the root crown while planting. With a clematis that age, these stems are incredibly fragile and can be damaged almost by just looking at them funny :-)

I would do nothing now other than provide normal care re: watering. If the wilting progresses to the entire plant and not just a single stem as it appears now, then it is quite possible that you have a case of clematis wilt and the vine should be cut back hard.

Neither issue is necessarily fatal if given proper attention but may mean you will have go without any flowering for a season or two.

that little puddle of moist soil may not be sufficient watering at transplant

Given the mix of happy versus wilted stems, I would agree with GG. If it was a watering issue the whole thing would be droopy.