There are two ways to go about this.
You can try to get the clown healthy so that he fights it off. This somwtimes times works but you will always always have ich in your tank. IMO and IME this is the wrong way to do it because anytime anything goes badly in your tank, the fish will get stressed and possibly have an out break. AND any new additions can really be susceptible to it because they are already stressed.
It is much better to get a quarantine tank and slowly lower the salinity to approx. 1.009. This kills invert and that is what ich is. Hence why you cannot do this in any tank that has corals, anemones, live rock, or inverts. Get a small tank, get a filter, put some pvc pipes and some cycled live rock from the display for filtration and then get all the fish in there. It takes approx 4-6 weeks. Then your display tank and the fish will be free and you won't have to worry about it anymore. The ich in the display tank will have died off because it had no fish to host.