There may be something that is stressing her in your household or she may have some sort of medical problem.
Does she pee often? Questions like : how old she is, how many cats with how many litterboxes you have may also give some more clues on wether it is stress or some sort of infection causing it.
8 cats is a lot--plenty of opportunity for some serious social and territorial stress! Do you have enough litterboxes for all of them? All boxes kept very clean? Enough vertical space and territory? Everyone get alone?
Cleo is sure trying to tell you something. Start with a check-up at the vet (to make sure it isn't a urinary tract infection--she could be in pain) and some environmental management if that's an issue. For more ideas and details, see articles in our free article library on litterbox behavior: http://www.littlebigcat.com/?action=library
Defanetly check with a vwt to make sure it isnt medically related.
How old is your Cleo? Has she been spayed? How many litter boxes does your home have?
If Cleo is not spayed, she could be peeing due to being in heat. Also, it is important to have a litter box for each cat ( a seperate box) I know 8 is alot of boxes, I would suggest it though. If space is a problem then try to get 4 extra large boxes. Make sure also that you are cleaning them each day, probably twice a day with 8 cats.
Well, then, you have your answer: not enough boxes, not clean enough, possibly a litter she doesn't care for. "Relatively clean" is not acceptable, certainly not with 8 cats.
Here's the test: you stick your nose 4" above the litter, take a deep whiff, and dig in it with your bare hands. Don't want to do that? Neither does Cleo!! And she is doing her very best to let you know...are you getting the message?
If fixing the litterbox situation does not work, then a vet visit is imperative to rule out urinary tract inflammation.
I really urge you to read through the articles at http://www.littlebigcat.com/?action=library. They're free, all you have to lose is a little time, but you could gain a great deal of insight along with practical suggestions for solving the problem.