Interviewer asking this question because You're going to come across people who are difficult in every office (ever). That's obvious. They want to know if you will handle those people in a way that is appropriate and professional or if it will send you into a tailspin and affect the quality of your work because the ability to work well with others is a critical skill for most positions. You need to just lay out the situation without getting into too many nuances or describing any office politics or "drama". Describe how you took steps to resolve the situation without losing your cool. This is really just explaining to your interviewer that not only did you solve the issue at hand, but you used what you learned to work with the difficult person in general.