How to clear my name after being Terminated under false pretense?

I was terminated due to false pretense. I had a target on my back every since I turned in my old boss for making it hectic to work for her and she was demoted. I was wondering what I could do and if I could file for defamation of character and to prove that I was the target of the four who are her best friends as well as her (my old boss). How can I go about clearing my name? I am not upset that I was let go, but because of what was said in the Corrective Action Conference form.