An example of collapsed sub menus within a mega menu is shown below. Here the “Development” sub menu is collapsed, and opens when its arrow is clicked:
To collapse a sub menu within a mega menu, go to Appearance > Menus and open the mega menu settings for the parent item. At the bottom of the Settings tab, enable ‘Collapse sub menu’.
Collapsed sub menus appear on click (not hover, as it results in terrible menu design). If you want the parent item to simply toggle the sub menu (and the link itself to not be reachable), enable the “Disable Link” option in the menu item settings.