The writing sample is a critical part of your clerkship application.  The clerkship committee is happy to help you choose your writing sample, but that is only the first step.  Once you choose your writing sample, you need to tailor it be an effective sample and polish it.  You want your writing sample to be pristine so that you convey to judges that you are the kind of detail-oriented person who will be helpful in chambers.  These slides have our advice on choosing and perfecting your writing sample.