Have you checked your .sims3.backup file to see if that was saved before you accidentally deleted Clarissas urn? The date and time between that and the .sims3 file can sometimes differ quite substantially. At the moment I have a Hidden Springs save file where the difference is twelve days, and a Sunlit Tides with eleven days. I'm not sure how this has happened, because usually the difference isn't that big.
If that little tip doesn't help you, the only other way I can think of is to start a new game in Appaloosa Plains and move your simgirl there and let her marry Calvin once again. If that doesn't appeal to you, I guess you'll just have to live without Clarissa from now on. I've never heard of anyone getting a deleted urn or gravestone back, so I don't think it's possible.
I have played the Riffin family several times in different games/savefiles, and then resurrected both Clement and Clarissa every time with the Oh, my ghost! opportunity and Ambrosia. What I can say about them is that they are very red when they come back to normal life again! Everything about them is red, hair, skin, eyes, eyebrows and all their clothes. I had to use testingcheats of course, and take them both to CAS for a thorough recoloring.
