This can sometimes be tricky.
My first suggestion might not sit well with everyone: Stock up the tank with snails! Just make sure they're a type the clowns can make a meal of. Can't beat fresh live food!
You can also try feeding them at night, once the mbuna seem at rest. Clowns are not fully nocturnal, but they are active at night.
Finally, don't worry too much about mbuna getting some of the clown loaches food. Very few mbuna are herbivorous, most are omnivorous, and some are actually insectivores. Most can handle a reasonable amount of flakes or pellets used for the clowns, as well as invertibrates like brine shrimp or insect larva. The main type of food you really need to completely forgo is worms, which are much too fatty for any mbuna.
