This is the perfect reason to have a competency model. If it’s not clearly defined, how can the person in the role possibly perform it with any degree of confidence? How can their manager help them to be successful if they don’t know what that person should be doing? It’s a sure way to create stress, frustration, and ultimately turnover.
So to create a competency model, either use a custom competency model development process (http://webcasts.td.org/webinar/2235), or start with a standard model or models that reflect the primary functions of that role (http://webcasts.td.org/webinar/1809). The webcasts listed walk you through the process, step by step, and provide you with materials you can download.