I need to create new custom site definition which inherited from Publishing Portal with all features and functionality. What is the best way to do this in Sharepoint 2013?
You can create a site definition with Visual Studio 2012 taking the onet.xml in
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\SiteTemplates\PUBLISHING\XML\
as starting point and add your custom features, modules, pages or whatever in it.