Command substitution - how?

[Xyzzy]

This is actually possible to some degree, see:

Renaming directory from mp3 metadata

I have a problem with directory renaming. I believe the solution must be something simple, just for the life of me I cannot figure it out. There is a tagged mp3 file in a directory (somedir\somefile.mp3). Goal: rename 'somedir' to 'artist - album',…