This is quite a common question for an interviewer to ask as they want to hear if you can describe a work colleague conflict and how you handled it. They are also looking to see how you handle working with changing personalities in a team environment. Talk about how you reached your ultimate solution and any lessons you learned from that. Describe the situation and how this person was difficulty. It is important to indicate the how/why they were difficult. Then describe how you dealt with the situation.
Example might include a difficult colleague with whom you had to work towards a common goal but he/she was not ready to cooperate. Did you both sit and talk about the issue and sorted it out? How did it go?
Try not to bring personal issues here and also not something which involved you as the one who created the issue. This answer is supposed to portray you as a positive person who is also a problem solver and not someone who is the problem creator.