Judging by How Big, How Blue, How Beautiful, they (she?) have been drawing inspiration from Brassed Off, as it's rammed full of tubas.
Slow and sombre, it's a little musical hallucination, with Florence intoning the title over and over and over. And, despite the impeccable orchestration, you have to assume it's a scene-setter or album intro, rather than a single.
Having said that, it does have it's own video, directed by Tabitha Denholm and Vincent Haycock, in which Florence dances with a doppelganger in a deserted amphitheatre.
Whatever the song turns out to be, it's nice to have them (her?) back.