It was quite interesting to search the blog for "clearly established". There are too many prior posts to list here. A cursory review shows that this is a key concept in habeas and civil rights litigation. It is typically used in order to limit claims for relief.
Thank you Scotusblog:
Title: Curry v. Bulter
Issue: Whether the Court’s ruling in California v. Cunningham (2007), which struck down part of the state’s sentencing scheme, was dictated by the Court’s ruling in Blakely v. Washington (2004) or instead announced a “new rule” that cannot be applied retroactively on habeas review.