I've seen roos and "oppositional" hens peck like heck on a lower rank chicken but it's usually on the back of the head or the back. Maybe put a light dab of Blu-Kote on the area? It can be a little too harsh so use your best judgement. And you don't want to get it in her eye. But the taste would probably stop the pecking. We have this concotion made by our local vet called Pink Wound. Its mild but really works. I've used it on every kind of animal you can name, including me. If the skin is not hot, or blistered or raw or scabby or etc etc etc, the feathers will problably grow back and you'll be left wondering what the heck happened. Chickens...gotta love them.
If she shakes her head, digs at her head a little it could mean ear mites.
Sometimes that's just where some chickens start molting and you'll learn their molt patterns if you observe and keep them long enough to see the cycles of the year. Here's a pic of an older NH hen that is starting her molt and you can see the areas she loses first are around her ears/neck and atop her tail... no picking in my flocks, so this is just part of a natural molt.