Muscular structure used when breathing (9)
![Ross](/clue/static/ross-mug.min.png)
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" .)