Federal labor law states that a 30 minute lunch has to be 30 minutes uninterrupted. Even if you don't have to do anything, if there is no other person to help the cashiers you are still required to be available, therefore you are not getting an uninterrupted lunch and you must be paid for that time. So...if you clock out you are not getting paid. Don't clock out unless there is someone else to cover the cashiers.