Clay Blast is a spell that requires a small piece of clay. When you cast the spell, the clay then grows and smacks someone in the leg. They must make a strength save against your spellcasting DC save, or fall prone. They must also take 1d8 bludgeoning damage. The clay can take any form you want, but the preferred form is a hand, so that it looks like the legs of one has been smacked. The roll to hit would be your spellcasting modifier, as one still needs to roll to hit. 

You can also target the spell towards a face. In this case, they must make a constitution save against your spellcasting DC save or get the clay in their mouth and gain the poisoned condition.