You can do this several ways:
Use the "Soft pause" feature. While the song is playing goto the Playback menu and select "Soft Pause". This will pause playback at the end of the song.
You can also get to this feature via the shortcut ctrl+shift+space or via command line with -softpause.
Using the pause here. You can right click the next song that will play and select "Pause Here". It will cause playback to pause when the playlist reaches the start of that song. You can tell what the next songs will be in Shuffle mode via right clicking the "next" button to bring up the future menu(a list of upcoming songs).
Create a new playlist with just that song. Create a new playlist with the "Add" button and drag and drop the song from the Media Library to it or right click the song in the Meda Library and select "To new playlist". Double click the song in the new playlist, Tuniac will only play files from within the active playlist. A playlist is "active" after you double click a song in it. If there is only one song and you have repeat modes off, it will play the one song and stop.
Use the text filter at the top right of a playlist(including the Media Library playlist) to filter out all other songs, eg type the song name you want. Tuniac will only play "valid" songs from a playlist, eg what is visible. You can remove the text filter to allow Tuniac to pick other songs again. If your one song wasn't the current playing track Tuniac will be forced to select it next because all other songs are filtered out.
Other things that might be helpful:
Holding alt while selecting a song will cause it to be played next, multiple songs can be selected like this. This queue is the first priority to use. Cancel by selecting a song without holding alt. (while this works I just found a heap of bugs in this code, I'll fix it later today maybe
Right click a song and select "Play next" to add to the top or "queue" to add to the bottom of the "queue list". This queue is the second priority to use. Cancel by going to the Playback menu and selecting "clear queue".