Probably a raised bed would be best. Fill it with good soil. Mix in some alfalfa and compost, etc. I do not have clay soil but I remember reading that if you dig a hole and refill it with good soil it will fill up with water and turn into a pool when there is a heavy rain. The raised beds will have good drainage and will not fill with water. Scroll down to Optimal Soil on the link.
http://learn.hostasdirect.com/...
