Ever hear a song that's just so reliant on a trope that you find it distracting, and find it difficult to take seriously? And then, on closer inspection of the lyrics, you find that the entire song seems to be about justifying its use of said trope? You've just discovered a Trope Justifying Song.
This usually happens for one of two reasons. Either the songwriter was aware that the audience wouldn't be happy with a slower/heavier/more cliched/more bizarre song than the songwriter is usually known for, and they attempted to work their explanation for doing so into the lyrics. Or else, they just realized the audience's potential reaction and decided to play it for laughs
. This only counts if the song is pointing out something that it does itself - attempts to seriously justify or to parody other songs in the artist's repertoire, or just the artist in general, don't count.
This could just be a Lampshade Hanging
. Plenty of examples on that trope's music page mention songs where a few key lines will make reference to some aspect of the song itself, but not all of them are cases where the lampshade is actually the main subject of the song. So perhaps it's a sub-trope of Lampshade Hanging
Could also be expanded to other forms of media, such as a Trope Justifying Film or Trope Justifying Episode, etc. "Trope Justifying Work" just sounded too vague.
One example that brought this potential new trope to mind is My Morning Jacket's "Slow Slow Tune", which is all about how slow the song is, how it's probably not the mood their listeners are looking for, and how it doesn't matter because the song is for the writer's son.