Hi Rose, so glad to meet you, and
to NGA.
Sedum do like some moisture, and clay is full of nutrients that would be good for your sedum. I think what I would do is add some bagged compost to the clay soil, mixing it well with the clay. On the down side of the slope I would build a stone wall bringing the area up to the same level and the higher part of where you want to plant the sedum. Hope that makes sense? Move your clay/compost mix to the back of the area and use thick plastic to line the inside of the rock wall, but not the bottom of the hole because you want good drainage. You can feather in the rocks as they come up to the level of the higher ground, that way you won't need them to come all the way around into a circle, but more of a half or 3/4 circle. And I would dig down a bit and plant the first stones down into the soil which will anchor the. You can also us a sharp gravel to fill in between the bigger rocks. The do the plastic liner and fill in with your clay/compost mix. Plant your sedum into the mix you have made and top dress with about 1 or 2 inches of pea gravel.
What kind of sedum are you using? It looks like it might be one of the Petrosedum rupestre?
