Clutch, if it exists, is absurdly easy to document. And anyone with minimal statistical skills can do it, and end this debate.
1) Define a clutch situation (doesn't need to be all that precise).
2) Are there players over a long span (50 games? a year?) who do better than others in these situations? (of course)
3) Do those players do as well (or better) in those situations than they do in other situations?
4) Are these results the same for other 50-game or season-long spans? (That's the important part.)
Make a nice, detailed chart of teh results and post them here. (You know, there are people who actually do such things.)
Then we have a basis for discussion, rather than referring to our Uncle George who once hit a rocket to left-center in his high-school semi-finals for teh league championship on an 0-2 count.