Good post, Bell. If we're going to talk about clutch what we're talking about is how a person responds to pressure. Some people wilt under pressure, for some it has no effect, and others excel in pressure situations.
What we're talking about is how that person handles the pressure. Since pressure is self-inflicted - that is to say that the pressure we feel is pressure we put on ourselves - we can't know how much pressure a player is feeling unless we ask the player and even then we can't be sure of getting an honest answer. We're taught at an early age to be modest. When's the last time you heard a player say, "Yeah... I was feeling a lot of pressure and I stepped up delivered"? Instead they say, "Thankfully I got a pitch I could handle and fortunately I was able to square up on it and it fell in for a hit.". That's BS, but it sounds better.
How we perform in any given situation is both physical and psycological, a combination of our mindset and adrenalyn, which contributes to our success. Until we can measure the effect of those things on each individual person's performance as well as the pressure they're feeling we can't statiscally quantify "clutch". We simply have to accept it as something that IS (or isn't, as the case may be).