Here's the blurb from Fangraphs:
"Clutch does a good job of describing the past, but it does very little towards predicting the future. Simply because one player was clutch at one point does not mean they will continue to perform well in high-leverage situations (and vice versa). Very few players have the ability to be consistently clutch over the course of their careers, and choking in one season does not beget the same in the future."
basically, its real, but not terribly useful.