If you want to make it appear after a click, yes you must use Javascript.
First, you have to make a class “submenu” inside of your class “item”. Then hide in CSS the class “submenu”
With JQuery like you said, you have to make a function like this :
$('.item').click(function() {
$(this).find('.submenu').show();
});
It will find the class “submenu” inside the class “item” you clicked and show it.
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solved Need sub-menu for vertical sidebar with CSS or Jquery