I was pleased as punch when I figured this out. okay. highlight the song you want to shorten in blue when in your library. right click if on a PC or ctrl click on a mac onto the song you've just highlighted and pick "get info", then go to the options tab and adjust the start and/or stop time. click OK or hit enter.
the iTunes help will have you stop there, but there's a way to make it permanent. if you highlight that song again and select "advanced" in the main window, you'll see "convert song to [some song file, mine is AAC]." if you convert the song to a different file the song will stay that short. so if you've got a song with some commentary before it (a radio or live recording) you can permanently cut it out.
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