I don't have this particular clematis but I know it is common this time of year in some of the other types...I call it brown-out. S A is a real vigorous vine so IMHO, you have nothing to worry about. :)

I also don't have this clematis, But my friend has a few types...
This heat that we've had could be the reason for the die out, I wouldn't worry unless the whole plant starts to suffer.

My Mom lost her WHOLE garden due to this heat we've had this year, but she doesn't have a sprinkler system and she's not about to go out in this heat with a hose!