1) There is a glitch that happens to me where he disappears. It's before the clock fight and only happens if you are linked with a dream eater. Not sure if it happens to anybody else but it has only happened to me when I am linked.
2) once you do a reality shift on the clock, you have to fight it while Young Xehanort phantoms attack you.
3) Young Xehanort is really Birth By Sleep's Mysterious Enemy. Look closely at his weapon when fight the clock.
4) The clock glitch seems to happen if people start attack the clock before the target symbol appears.