Kirk McElhearn, on the iTunes Apple Music cache:
If you use Apple Music on your computer, you’ll find that the more you listen to music, the more your hard drive’s free space dwindles. Even if you don’t save music for offline listening, iTunes stores what you listen to in a cache. There’s a good reason for this: when you listen to something again, you don’t have to re-download it.
However, if you listen to a lot of different music, and don’t have a lot of disk space, then you may find that your free space is shrinking. It’s easy to clear this cache, but you can’t do it from within iTunes.
Read on for the details. It’s short/easy, but non-obvious.