As far as I know, a clean game has a mark in each of the 10 frames of bowling. If, in the 10th frame, you threw a X, 9, miss, that should still be clean, since the 10th frame had a strike.
We talk of the 11th and 12th frames because we need some reference for the 2nd and 3rd ball thrown in the 10th frame, but technically there are 10 frames in each game.
Only because the scoring rules say a strike is 10 plus whatever you throw in the next 2 balls, do you get to throw 2 more balls in the 10th frame. Same for a spare: you already threw 2 balls; the next ball is needed solely to determine the score.
To be safe, check the "ABC" rules on their website to see the formal definition of a clean game, if there is one.
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