First of all, let’s define what an asset means. Usually by a valuable asset an employer means someone who exceeds their expectations. That means that hired employee needs not only to fulfill the requirements from the job description, but also takes additional projects and responsibilities. So to become a valuable asset you will need to show your dedication in action, constantly evolve and take an active part in the company life. Being proactive and offering new solutions to improve processes is another key to success.
Answering this question show your eagerness to develop your skills and take more responsibilities than the position presupposes. Bring some examples from your previous career, demonstrate that you are capable of contributing to the company’s success.