MikeAqua:Kango, shovel, 2 wheelbarrows and 4 people would do it in a day, assuming you have an easy way of disposing of the clay you remove.
I was going to suggest kango + dingo, but similar idea. You've really got two problems, breaking up heavy clay and moving heavy clay, so you can automate one or both. Pick, shovel, and wheelbarrow would be the most painful option, neighbour did that to dig out a bank for his driveway and spent several months on it, a few hours each weekend, until eventually he got a guy using a digger up the road to come over and do the rest in about 30 minutes.