How to plant a fig tree in really heavy clay?

I have a young fig tree (ficus carica) in a pot which is very bushy, growing rapidly and which would suit my sunny brick wall, espalier fashion.Trouble is that my soil is so heavily clay that I could literally throw a pot with it. Despite loosening it with grit, compost etc it is still almost impossible to dig in the summer. Can someone give me planting instructions for this tree please and how to prune it? Many thanks.