Talk to the hiring manger about a time when you have displayed a strong ability to handle stressful situations. You should use a recent example, if possible, and be sure to talk about the positive outcome of your actions.
"As a Stock Broker, I am exposed to many high-stress situations on a regular basis. One of the most high pressure situations that I can think of was just last month. One of my largest accounts wanted to take his portfolio elsewhere, as he had a friend who recently finished his MBA and was joining a competing firm. There was a lot of pressure on me to save his business. We met, and I helped him to see that, friendship aside, our firm was the best choice for him. Losing his business would have had a huge impact on my portfolio so I was incredibly relieved to have saved it."