A single bird is a very difficult introduction to do. All of the other birds know this one does not belong and will attack, often times relentlessly.
Pull another chicken from the established flock, and let her and the new comber become a pair. It may take a couple of weeks. Try the see and don't touch for a bit. Once they get along well, then introduce the pair to the flock, or even add another bird, and then add the trio to the flock.
Lots of hideouts, escapes, multiple feed dishes and waterfowls.
Mrs K