Kitty litter is basically dried clay (soil/dirt). Imagine what you would do with a clump of gray clay on your floor and do the same. The best thing is to remove the clump and you can do that by removing the trap and getting as much of the clay out as you can. Running water would be a help but only if you can keep it running. You may be able to use a plunger to help with this but I would not count on it.