One of the first apps I ever bought for Mac OS X was MP3 Trimmer. I have just used it in Sierra (bought it when using Jaguar, 10.2) to trim a section of a Mike Oldfield track. It was a particular 6 minute & 5 second section from Hergest Ridge Part 2, from 9'31" to 15'36". My version of Hergest Ridge is from the Boxed Collection (I have both CD & vinyl versions) and when I imported the trimmed selection into iTunes, it was magically given the original Hergest Ridge album artwork. I'm pleased with that result.
The process can be done with QuickTime X but it's a two-stage operation & results in a .m4a (AAC) instead of the desired .mp3 which can be burned to a CD for use in the car.
Open exported .mp3 file in QT X and select "Trim" from Edit on the menubar & save the trimmed file. This will be in .mov format. Now open that one in QT X & export the audio. Oh, look. It's a silly .m4a file that needs more conversion. With some preference-fiddling in iTunes, it can be converted into .mp3 format again but that's an extra four steps.