get first instance of element

I need to get the VAT_TAX_RATE from this XML data, but it's within <?for-each g_lines?>, I only need this value ONCE, not every line, for a field outside of the for-each area.How do I get it?

<G_HEADERS>
   <LIST_G_LINES>
            <G_LINES>
                 <LIST_G_SHIPMENTS>
                       <G_SHIPMENTS>
                            <VAT_TAX_DESCRIPTION></VAT_TAX_DESCRIPTION>
                             <VAT_TAX_RATE></VAT_TAX_RATE>