Clover can be killed with repeat applications of a combination selective broadleaf weed killer. Be sure to use a three-way herbicide such as Trimec or another brand that includes the same ingredients. Any product will do if it has two or three active ingredients from this list: 2, 4-D, dichlorprop, MCPP (mecoprop), dicamba, triclopyr, or clopyralid. (Labels always list ingredients right up front, so you can easily check to see what's in a product before buying it.) Choose a spray; it's generally more effective than a granular weed-and-feed product. Carefully follow the instructions on the label.
The clover should be healthy and actively growing when you spray it so it will readily absorb the herbicide. Add a squirt of dish soap to the solution to help the spray stick to the leaves; otherwise, it may bead up and roll off. Make sure you spray when no rain is expected for the next couple of days, and avoid mowing or watering for at least 24 hours after application. Even with a thorough spraying, it's likely that you'll overlook many clover plants, and a few will manage to grow back. New ones will continue to germinate, too, so count on some follow-up applications. But it shouldn't take too long to bring the clover to a tolerable level.