You should build your mesh so it works well with the existing weight painting of the avatar- move your collar until it is located where it will only be in the area that is only affected by the collar bone. If it won't work well you can also break the collar off the rest of the mesh, copy only the collar bone to the collar, and then put it back in (sorry if it's not called collar bone, maybe it's clavicle, I mean the bone BETWEEN the neck and the shoulder/upper arm, that one is not bendy)