I would be more concerned about the belly and body shape than the tail.
I would say the fish has a growth in its body that is causing the swollen belly, bent back and general poor health, including clamped tail.
Check the water quality and maybe do a 75% water change and gravel clean the substrate every day for a week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if that hasn't been done in the last 2 weeks.
If the fish is still eating, vary its diet.
I don't think there's much you can do apart from keep the water clean and when the fish stops eating, euthanise it.