I would target feed the Clown until it gets some size back. Just a couple small pieces of food, but as often as you can. 4 or 5 times a day even but just a couple pieces. Frozen Mysis Shrimp or minced Scallops are an excellent food to put the weight back on.
What happens sometimes with planktivores is that when they get a big feeding their body dumps any undigested food to make room for the fresh incoming feast.
This causes two problems;
1 - the nutrients that are not quickly absorbed through the intestines get dumped. Over time this can cause the malnutrition your fish is experiencing.
2 - The expelled food causes nutrient problems in your water.
Try to pay attention to it when it eliminates to see if it's normal (not white or stringy).
Once your Clownfish is back to normal just feed small amounts twice a day.