I don't know if it's quantifiable but it exists. Anyone who's played organized sports, at pretty much any level, knows there are players who want to bat when it matters and players who hope the guy in front of them does the job, players who want the ball and players who want anything but to be near the ball. In truth, my belief may be in "unclutch" rather than clutch. If you're thinking "OMG, I hope the game doesn't come down to me", you're probably going to fail. If you're thinking "I just hope I get the chance", which is just standard gameplay in my mind, you're likely to perform to your ability.