Click To Open
You can use the ClickToOpen property to specify that menu items do not expand when the mouse hovers over them until the user clicks the menu with the mouse. When ClickToOpen is True, the menu does not expand until the user clicks on a root item, at which point the root item expands. Once clicked, the menu expands and collapses as normal, until the user clicks again (either on a menu item or outside the menu). The ExpandDelay property controls the time, in milliseconds, after the user first clicks the menu until the menu item expands. The CollapseDelay property controls the time, in milliseconds, after the user clicks a second time until the menu items all collapse.
<telerik:RadMenu RenderMode="Lightweight" ID="RadMenu1" runat="server" ClickToOpen="True">
By default The ClickToOpen functionality is applicable only for the Root items of the RadMenu. To apply the same functionality for the sub-items, a custom implementation is required. An idea how to achieve that you can find in the Apply ClickToOpen functionality for sub-items of RadMenu knowledge base article.