I am trying to find out what this song is called I have a 10 second clip and the app Shazam cannot pick it up.
It’s a rock type song at 0:50 on this video from the movie Under the Lake.
This is a piano cover of Nirvana's Smell Like Teen Spirit.
It is confirmed by this review in Variety:
Sam is led to the home of The Songwriter (Jeremy Bobb), who turns out to be…the secret mastermind behind all pop music since the dawn of the counterculture. He wrote “Smells Like Teen Spirit” on the piano!