It depends which part you are answering. In Part 1, the examiner is able to repeat the question but will not offer you much help beyond that. In Part 2, you can ask for clarification during your one minute of preparation time if you need to. It’s not a good idea to ask questions during the two minutes of speaking time as this will disturb the fluency of your response. In Part 3, the examiner is able to offer much more help and can paraphrase the question for you. You should try to ask checking questions such as Do you mean…? Rather than simply say Sorry, I don’t understand.