Hi,

You can create a submenu under the start menu using the following code. Here I create a module for customer details report so I am going to create a menu and submenus for customer details report. So we will create a menu for customer details.
<menuitem id="menu_customer_details"
            name="Customer Details"
sequence="2"/>Now we are adding a sub-menu so, action is added for the sub-menu.<menuitem id="menu_customer_details_main_reports"
         name="Reports"
parent="menu_customer_details"
         sequence="0"/> To create a sub-menu, a parent main menu will be required. Here the parent menu is menu_customer_details.

<menuitem id="menu_customer_details_report"
         name="Customer Details Report"
parent="menu_customer_details_main_reports"
action="action_customer_details_report"
         sequence="0"/>
Next, we are going to define action,
<record id="action_customer_details_report" model="ir.actions.act_window">
   <field name="name">Customer Details Report</field>
<field name="res_model">customer.details</field>
<field name="view_mode">tree,form</field>
   <field name="view_id" ref="customer_details_tree"/>
   <field name="help" type="html">
<p class="o_view_nocontent_smiling_face">
           Generate Customer Details
       </p>
   </field>
</record>

Hope it helps