The clown looks emaciated, possibly internal parasites. The clown also has what appears to be some curvature of the spine, genetic issues. It was a young clown and it's not uncommon, I have been duped into buying a less than ideal specimen before as well.
In the future really make an effort to quarantine your new live stock. If you can not quarantine then do not buy such young clowns. I have a feeling this clown was probably doomed from the get go.
Always always check for ammonia,nitrites and nitrates when trouble shooting the issue. Make one change at a time, a massive water change or dipping the clown in FW will only stress it more. Subtle changes and then observing for a positive response prior to the next intervention. You'll learn a lot by waiting to see which intervene was beneficial rather than haphazardly throwing the kitchen sink at the tank.
I would wait a week or two before attempting another clown, and choose carefully when looking for the next clown. The easiest clown to catch at the fish store is not necessarily the healthiest.