I recently wrote about how you can clear the Apple Music cache on your computer if Apple Music is eating up your hard drive space. You can also do this on iOS devices, though it’s a bit more radical.
Here’s an example. My iPod touch is running the iOS 9 beta, and it uses Apple Music and iCloud Music Library. There was 7.1 GB available on the device, with about 1 GB of music that I had downloaded for offline listening.
Go to Settings > Music, and turn off iCloud Music Library. (You don’t need to turn off Apple Music.)
After doing the above, the device now shows 4.6 GB free. You can turn iCloud Music Library back on again, to start being able to sync your device with your iCloud Music Library on other devices, and to be able to download files for offline listening.
So, in addition to about 1 GB of downloaded files, there was about 2.5 GB of cache files.
Curiously, turning Apple Music off and on doesn’t clear the cache, only toggling iCloud Music Library does this.