Flowerbed Soil

I had some work done on the main water line to the house last fall that involved having a six foot deep trench dug in the middle of our foundation flower bed. We’ve got heavy clay soil and when the material was backfilled, I ended up with a pretty thick layer of clay on top. Now it’s hardened up and the hosta that was transplanted in that area has died.

I know I have an issue with the soil. How deep would I need to remove the clay if I then replaced it with good soil? I just need it to support some flowers and hostas.

Or am I better off removing some, adding compost and tilling it all up? I’m thinking I’ve got to remove a good amount of the clay. I’m not sure if tilling in compost will be enough.