This card is an additional action space. A player who uses this action space must pay you 1 food and receives 5 clay. If you use the Clay Deposit yourself, you may choose to take 2 bonus points instead of the clay.
If you use the Clay Deposit yourself, you do not need to have or to pay any food. [ref] The Clay Deposit counts as played improvement, e.g. for the Village Elder or as prerequisite for the Carp Pond. [ref]
It's a quite powerful card... In a solo game I got 86 points
It's far more powerful in solo games though, since there you have lots of spare actions while you wait for resources to accumulate. Even if you could get the resources you need without taking more actions in a 2+P game, going to the Clay Pit frequently takes pressure off your opponents.