A waiter clocked in at nine but only started at ten. Can I fix it, and will it look like I hid something?

You can fix it, and it will not look hidden. Open Reports → Hours worked, find the row and press Correct. Change In to ten, write a Note saying why, and press Save the correction. The row is permanently marked as edited, keeps its note, and the change is recorded against your account. Only the owner or a manager can do this. That is deliberate: an honest correction and a quiet one look completely different, so a dispute later has something to read.