If you've already got the clover planted and it's being invaded by weeds try poast. It will kill weeds but will not effect legumes.

Whitetail institute sells a product called Slay and Arrest. I think Arrest is the one you want but take a look http://www.whitetailinstitute.com/products/herbicides/

I got some at a local agway. I'll check the package name and post it as soon as I can.