Degloving is a defense mechanism for rats, similar to a lizard/gekko. The whole tail is not dropped (like the aforementioned lizard/gekko), but rather the outer layer of skin tears away from the tail bone. Its very painful for the rat, and can occur when the rat’s tail is trapped and they pull away, or by picking the rat up by the tail (which is a big no-no!). This injury can be very prone to infection and should be treated correctly immediately.
What must be done?
If the injury is extensive, wrap the tail in a clean cloth and seek vet assistance. The vet will advise what needs to be done (amputation is a possiblity). They might also prescribe antibiotics and/or pain medication.
If the injury is minimal, then wash the wound with a saline solution (a solution that is least toxic to sensitive tissue although it is not an antiseptic solution), which you can make from 1 teaspoon of salt in 470ml lukewarm water. Dry it by gently patting it with a dry towel, and apply an antiseptic cream (like Savlon). The wound should dry up and heal (but the lost part will not grow back).
While the wound is healing, avoid litter type bedding, and use cloths (not those that flake) and ink-free paper towels. All bedding must be changed daily! If the wound isnt healing, contact your vet for further information and help.