Some companies have a more open and transparent review process. I'm not sure if it would help with a clique, but at least people have to put information in writ

Some companies have a more open and transparent review process. I'm not sure if it would help with a clique, but at least people have to put information in writing which when push comes to shove can be verified. And, reports can review their managers. If your management chain doesn't care then yes, senior people walk.