I have a full grid and understand all the wordplay, one row where something has happened, the 18 extra words to be sorted into 3s (with five known synonyms) and even the message from the extra letters, which does not suggest anything to me.
However, I have no idea of the song or the singer. I suspect that this is way out of my comfort zone, having not listened to pop music for over fifty years. Perhaps I ought to go and consult someone a bit younger and more hip (which is not difficult to achieve).