|Managers are often advised to hire strong employees to build a solid department and to increase their own chances of being promoted. The idea is that if a department has bench strength, the manager can be promoted and someone within the department will be ready to step into the vacant position.|
At the same time, some managers fear hiring strong people because such new hires can make the manager look bad and even lead to the demise of the manager. It sounds like you had no fear of hiring a strong individual, and now you are picking up some cues that she may be strong in the wrong areas.
At this point, all you have is a feeling. You have no hard facts, no evidence, and no real data. Perhaps she is subverting you, but perhaps she is not. The best approach is to be far more observant of what is going on in your department. If you see any acts of sabotage, subterfuge, or insubordination by this new employee, go immediately to her and then, if necessary, to your manager.
If this new employee has dreams of becoming a long-term employee, this approach should wake her up.