If you're not absolutely certain that you understand the question, ask for clarification. This will always be better than giving an unsuitable answer. If you have developed the habit of taking a second or two to plan your answer, this should further help you avoid misunderstandings. In a Calm and polite manner simply ask for them to clarify the question or perhaps rephrase it for you. Most interviewers will oblige once or twice with such a request, but any more than that will lead them to think that you have a communication problem.