This functionality is currently only available for extras associated with organisations and sponsorships.
To create an extra that can only be answered by an administrator you must set the extra to admin only.
This is done by selecting the option labelled ‘Make item admin only?’ on the add new extra form.
To allow the answer for that extra to be read but not editable by the user you must also check the ‘Make admin only answer readable by users?’ option on the same page.
If the admin extra has been set as readable by users, it will appear when they edit their extras but will not be editable.