Has this always been a problem, or is it progressively getting worse every year? Clover's normal lifespan in a plot is 4-5 years before you need to do something. When the weeds start to take over the plot, you basically need to start over. Till it under, and plant oats in the spring. Let the oats grow to maturity which will be about July then mow them down. Go in and broadcast your new scratch it into the soil with a disk, harrow, hand rake or whatever. Some of the oat seed from the previous crop will grow as a cover for the new clover and stay green into the fall. The deer will browse on the oats all year, and the clover will start establishing itself in the fall. By the next spring, the clover should be able to take over the plot itself.

This probably isn't what you want to here, but it's just a fact of growing forage crops. If you live in an area that a lot of hay is grown, pay attention to the fields. Every few years the hay fields will be turned to corn. Farmers that raise livestock and grow grain and forage crops have a crop rotation program to maximize the output of their land.

As far as chemicals, unless you have some new Roundup Ready clover, you will kill, or at least stunt, the clover when you spray for broadleaves.