jQuery – Toggle megamenu submenu items by click

Try this ->

jQuery(document).ready(function($) { 
 $('#menu-main-menu a').click(function( event ){
 var visibleSubmenu = $('#menu-main-menu ul.sub-menu:visible');
 var submenu = $(this).next('ul.sub-menu');
 if( submenu.length >= 1 ) {
 event.preventDefault();
 visibleSubmenu.slideUp();
 submenu.slideToggle();
 }
 });
});