Geezus man, the definition may alter a little but it isn't as freaking difficult as you are making it.
For me: 1.Of course a rare level of talent but several talented players don't have that extra " it".
2. The refuse to lose mentality. Yes, everyone loses from time to time but you can see those that just can't except it until the final whistle/buzzer, whatever. And they seem to hate that feeling even more than those around them. And just will never concede anything.
3. And probably most important. When the pressure and adrenaline actually up your game. There are many talented players that have great games, WITH the team but when everything rests on them for that final shot final catch, throw or defense, they feel the pressure and it's a 50/50 shot.
While those with "it" somehow thrive off of that pressure and their game improves.
You add all 3 of these, there you go.