This question is set up to test your communication skills. A good approach would be to first give an example of when a supplier might be difficult before saying how you will deal with it. Then give an example of when you have done this in the past to confirm the point, even if not with a supplier but an instance of conflict in the workplace which you resolved. Key aspects of good communication skills when in a difficult situation are to be firm, honest and empathetic but remain assertive.