To help keep you on track, really listen to the question. Repeat the question to yourself, out loud if you want. Take a sip of water to buy yourself a moment to gather your thoughts. Don't just start answering if you are not sure what they are looking for.
It's ok to ask an interviewer to repeat a question or to clarify the focus of a question. It’s really important that you answer the question you are being asked, so asking for the question to be repeated or clarified is absolutely ok and interviewers won’t mind doing this. Better to ask than to answer the wrong question.