If this has been going on for a while, rather than a temporary social order shakeup, constant bullying can sometimes result in the erosion of the victim's self confidence. This converts the individual into a perpetual victim, in turn causing her to behave like a victim which encourages more bullying.
If this has only been happening for a few days, I wouldn't advise intervention. Give your chickens a chance to work it out.
If you've noticed this bullying going on for a few weeks, though, intervention could restore the little hen's self confidence and that would cause her to stand up to the bullies instead of backing down as she does now.
Having a run large enough to partition a section will help. I had a hen such as this who spent her days relaxing away from the torment she had been enduring, and three weeks later, she emerged transformed.
The secret is to have the safe pen in full view of the flock, place the hen in it each morning, allowing her to see the others and be seen by the flock, but not have to put up with their bullying. Then at night, let her go in and roost with the others. This preserves her membership in the flock while giving her a vacation to recover her self confidence.