Hi Malte,

what you will need here is some kind of 'anchor' or fixed startnode for your submenu(s). If you know for instance that your submenu should always start at nodes with level three you can do the following:

<xsl:variable name="subMenuRoot" select="$currentPage/ancestor-or-self::*[@isDoc and @level=3]" />

Or if you know that the submenu root will always be of a certain document type you can do:

<xsl:variable name="subMenuRoot" select="$currentPage/ancestor-or-self::*[@isDoc and @nodeTypeAlias='your-node-type-alias']" />

Once you got the root node it gets easy:

<xsl:for-each select="$subMenuRoot/descendant::*[@isDoc]">
  //output the nodes, e.g.
  <a href="{umbraco.library:NiceUrl(./@id)}">
    <xsl:value-of select="./@nodeName" />
  </a>
</xsl:for-each>

Hope that helps!
Sascha