How to just play one song in iTunes?
Following assumes that:

you want to listen to one song in your library
you know the name of the song (or part of it)
Spotlight is indexing your music (by default it's enabled)

Invoke Spotlight, enter song name, select it from suggestions (it may already be selected by default), press enter and listen this one song (in Big Sur it's Music, but it should work in similar fashion with earlier versions with iTunes as well).
Scenario using keyboard:

⌘+Space (to open Spotlight search)
enter song name (like 'Wasted Words')
music file should be displayed in results (maybe even selected as default)
file selected press Enter (starts playing song)

If there are too many files found one can narrow search by adding kind:music right after song name. It is also useful then one remembers only one or more words from song name. Just enter word(s) and narrow results down with kind (like Words kind:music)