Im looking at the picture and I am not sure its missing, the pic on the left seems to have a little black at the fin location, which would be the fin, just clamped against the body. Its a juvenile, right? Fish looks healthy otherwise, its swimming straight, doensnt look emaciated. If it helps any, my clown does that sometimes, and other times it lays on the bottom like its dead, and its been doing it for 13 years. I think this is one you should give some time. If its eating, just let it be and observe.