"To lead the department" means to be in charge, responsible, and have authority over a particular group of people who work together in an organization. Full definition
Part of his goals for earning a good review and perhaps a raise in his salary was to lead his department in prayer and heal the sick. (religion.blogs.cnn.com)
Led department managers to create effective budgets and use monthly financial reporting to analyze their outcomes and to identify cost efficiencies. (jobhero.com)
I have succeeded in leading departments forward to improved maths results. (tes.com)