This happens only when connecting to certain digital receivers, and it usually only happens to the first song in a stream. These digital receivers are simply synchronizing their clocks with the music stream, which may take a few seconds. This is normal when using such a receiver.
If you change the playlist or select a new track, the stream will get clipped again when the new selection starts. It will also clip if you pause and resume the same song.
Advanced tip: You could make a very short silent track that's the same length as the clipping you hear, then use it as the first track in your playlists.
In the event that you hear clipping between songs but haven't done anything to cause it, try turning on the Crossfade option. To access it: From the iTunes menu (Edit menu in Windows), choose Preferences, then click the Audio button.
The only time you should not try Crossfade is if you are listening to DTS or Dolby Digital 5.1 (AC-3).