Practicing your answers to some possible interview questions really helps you to prepare. They won't always ask this question, but for an administrative assistant's job it would be a good question to ask. We certainly know pressure. This is the answer I normally give to this particular question.
Do you work well under pressure?
I handle working under pressure by being organized. I prioritize what I need to accomplish and as I complete it I cross it off my list. I maintain a to-do list for all projects.
If you are already an assistant I am sure you have had to handle stressful situations. How have you handled them? Do you remain calm? Are you organized? Think about those things and come up with your own answer to this question and your own examples.