The overview page gets created automatically when you add a new page in the main navigation. But only when there's another, child page that sits below it in the navigation.
You can hide these overview pages so that only links to the child pages appear in the menu.
Hover over 'Structure' then 'Menus'. Select 'Main menu'.
Find top-level page in the main menu list and select 'edit'.
Change the Path to <firstchild> and save it.
Get help
If you need help with your Drupal website, contact the Drupal team via webmaster@admin.cam.ac.uk.
Content editors around the University can get guidance and advice from the Content Community.