what does 'clean' mean in the itunes podcast title??

The 'Clean' title is the opposite of 'Explicit'. It's an option that the podcast publisher can add to the feed to indicate that the episode does not contain any offensive content, or that there is another version that contains offensive content but that the episode you are hearing has been edited to remove that content.

Of course, adding that tag is completely up to the podcaster and it may depend on what they consider clean. If you see a podcast that is abusing the tag, you can report it using the 'Report a Problem' link.