My filly used to do this, what you need to do is some gentle tail movement exercises.
Stand near her tail and close to her body, as if you were going to brush her tail, make sure you let her know you are there by moving your hands around the area. Then you gently grasp the tail and move it up and down and then from side to side.
There will be resistance and she may clamp down even more to start with you have to work slowly and very gently and do not rush the movements.
Stop for a minute and then try again. You can repeat 5 times every other day to start with.
Eventually you want to actually move her tail gently in a circular motion clockwise and anticlockwise.
Gradually over time she will relax and feel more comfortable.