At one time a mixed white clover and grass lawn was desired. I have one. The clover holds up well in heat and dry weather as you’re seeing. The clover also gathers nitrogen from the air and shares it with the grass essentially helping feed the lawn. Do a web search for “the benefits of a white clover lawn” That said, if you want to kill it, you can spray it with Bonide Chickweed and Clover Killer. You’ll need to wait until the lawn is well hydrated (not just a quick watering or shower but a deep soak) and until temperatures are below 85° for several days. Applied in high heat or onto a dry lawn, the weed killer will also kill the grass. Unless the weather pattern changes or you have the ability to water heavily, you’ll have to wait until mid-August or so to kill the clover and other weeds. In the meantime, you might learn to love the clover lawn as you watch it stay nice and green as the grass withers in the drought.