I would try the dispersal method with good food and let them have at least five minutes worth of food around the tank for a new fish.
I have a 3 1/2" Regal Angel in the tank for a year and a half which started eating with this approach.
Mixture of various frozen foods, freeze dried mysis and brine shrimp mixed with seaweed pieces and "Sea Veggies" in a 1/4 measuring cup with Selcon and some aquarium water. I then turn the my protein skimmer and two 3,250 gph Hydor 8 water pumps off and dip the measuring cup with the food into the water. The fish start to eat it right out of the measuring cup before I empty it into the tank and it disperses which gives everybody a chance of tasting and/or smelling some food. This dispersal method for a few days may require an extra water change or not depending on how much and how often you try it.
Drs. F & S lists the Regal Angel as well as the Clown Tang as "expert only" level in a range from easy, moderate, difficult, and expert only.
I have just received a 4" Clown Tang on Friday evening and tried my dispersal feeding approach Saturday evening before the Clown had a chance to come out of its back right corner territory and join the Powder Blue, Vlamingi, Purple, Sailfin, and Yellow Bellied Regal tangs. It did swim out and at least sniffed some food which is approaching my goal to get a "new" fish interested and eating. This morning before the lights turned on the Clown tang was swimming around with the other tangs with no aggression shown.
I hope it will join the other tangs and my Regal, Blue Girdled, Lamark, and Red Striped Dwarf Angels for dinner tonight!
P.S. I love those Flame Angels although I have never had success with acclimating one for whatever reason so I am trying a Red Striped Dwarf which came with the Clown.