I think the best way to go would be to go with a raised bed. If you are going to dig down to mix in components to make it more faster draining you still have the issue of the fact that the surrounding soil will not have been amended and you will have created an area where water will tend to pool because it cannot drain away. Creating a raised be solves that issue and also will allow you to use whichever soil you want without having to deal with the clay rich soil....
