no, we dont have to mention the meanings of the idioms....
so even if you do know the meanings, you are advised to proceed directly to sentence-making because
1. writing something which is not required is construed as over efficiency and that is generally disliked by the examiners.
2. it would amount to time wastage..& with precis, paragraph and other questions to attempt, you wont have much time to spare
3.what if you think you know the meaning but you are wrong?? people can make correct sentences even with vague or even wrong sense of the idiom ..there is no need to alert the examiner that you are firing blind