When someone says Sleep Clause, they mean they have a rule about putting pokemon to sleep when you're battling. In other words, you can only use a sleep attack (like sleep powder, spore, Dark void, or etc.) to put a pokemon to sleep in battle only once for the entire battle. If you miss, you can use it until you're able to put one of your oppenents pokemon to sleep. But you're not allowed to use it more than once to put the pokemon a sleep again. That's what they mean by Sleep clause.