The most effective selective weedkiller still available is Verdone Plus.

Unless it already has some in it, you may need to mix it with a surfactant, such as organic dish soap, to get it to stick to the leaves. Clover is notorious for sprays just rolling off the leaves!

I had patches of clover in my lawn and bought a reputable feed and weed product from a garden centre and it cleared it all. Hope this helps