Hello peterpt,
And welcome to Apple Discussions!
You can burn portions of songs by changing the Start/Stop time of each song. To do this, locate one of the songs you wish to burn a portion of, right->click on it and choose "Get Info" from the Shortcut menu. From there, head over to the options tab, where you will see two different boxes for adjusting the start and stop time.
The burn CD will only burn the song from the time you specified it to start as well as the end. Just make sure that if you want to hear the whole song after that, to change the settings back to how they were originally.
Hope this helps.
B-rock