If the tail has been injured and torn off it will almost always grow back provided you supply good water quality and maybe add a bit of salt to the water...probably 3 tablespoons for every 10 gallons of water.
There will probably be a slightly visible line where the rest has grown back (my Crystal has one where some fish at the pet store ripped a good bit of hers off) but it will not impair the fish's ability to swim.
Sometimes fish can have one side of the double tail shortened/bent because of its genetics; there is nothing you can do about that - just it for who it is.
I would not advise gluing anything onto your fish. My guess is another fish or a decoration in the tank has caused your fish to have its tail torn off. When the tail starts growing back it will be a bit clear, then the color will fill in after a few months.