Crabgrass and weed preventer prevents weed seeds from germinating but won’t kill clover or any weed that is coming up from and established root system.  As the name implies, it is a weed “preventer” not a weed “killer”.   To kill established clover and other lawn weeds, you’ll need Bonide Chickweed and clover KILLER.  It will need to be applied when the weeds are actively growing…usually mid-May or so.