You can do this with iTunes. Here are the steps:
1) Choose a song and open the "Get Info" dialogue through the File menu or by right-clicking.
2) In the "Get Info" dialogue, go to Options and set a start and stop time for the song. Close the dialogue.
3) In the Advanced menu, choose "Convert song to ***" where *** is a format (the default is AAC but this can be changed in your preferences).
This will make a copy of the song from the start time to the stop time.