We must have the most clutch team in the league, right? Kyrie is known as possibly the most clutch player in the league and he's not even playing. I don't think Hayward exactly has a 'Paul George' reputation either.
I was going to say that I was a little uncertain about Jaylen, but he literally just hit a step back three at the buzzer to win the game like a week ago. His focus and determination make me think that he will only get better in this area.
Everybody else mentioned - Terry, Horford, Smart, Horford, Tatum - all totally amazing clutch players. That is a total of 8 players who have a killer mentality. I think it is clear the types of players Ainge and Stevens target.
Edit: Heck, I even forgot about Morris. That's 9 right there!