From what I can tell, that attribute is defined by XML when the song/podcast is loaded into iTunes, either from an RSS feed or from the iTunes Store itself. My guess is that it's stored in the
iTunes Library file (and mirrored in the iTunes Music Library.xml file).
This line is from the RSS feed for the Wordpress podcast:
<itunes:explicit>clean</itunes:explicit>
This line is from iTunes Music Library.xml file entry for that same Wordpress podcast:
Since there is no way to edit the iTunes Library file outside of iTunes itself, there's no way to manually set a file as "explicit" or "clean."