I have only ever heard it as CLAH-ra (in the
UK), never Clarra (as in
Clarence or Clarrie). I saw a film once where the name
Clara was pronounced CLAIR-a, which I think is how it's said in
America. I could still see Clarrie being used as a short form of
Clara though as diminutives don't always follow the same pronunciation as the full name.
Incidentally, I had a feeling this post would cause confusion with the American posters!