Clemson winning the ACC would likely make them #2 seed in playoffs, with ND likely getting the #4 seed. That's assuming, of course, that Ohio State plays and wins the Big 10 championship game.
If OSU does not qualify for Big 10 game, they may not make the playoffs. In that case, so many things could happen.
If ND wins the ACC game, Clemson is out and ND may be the #1 seed.