Muscular structure used when breathing (9)

I believe the answer is:

diaphragm

(Other definitions for diaphragm that I've seen before include "Flexible membrane" , "Partition between thorax and abdomen" , "It separates the thorax from the abdomen" , "Partitioning membrane" , "Midriff" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.