It can be argued that the most important task of leaders in business is to serve customers. A leader must first and foremost maintain a customer focus for themselves, the team that reports to them, and their entire organization, leading by example.
Who are the customers? Any individual who use or receive their services or products, be it paying business customers or, for example, the constituents that leaders of governments serve. Effective leaders work with clients and customers to assess their needs, provide information or assistance, resolve their problems, and satisfy their expectations. As part of their organizational knowledge, leaders know about available products and services, and are committed to delivering the highest quality results possible, given the situation.
For the most part, leaders serve customers indirectly. Here are two possible measures: Did you make one decision today that will directly impact customers in a positive way? Here is perhaps a stretch goal: Did you interact directly with at least one customer today? Yes or No?