Anatomical air bag (4)
I believe the answer is:
lung
'anatomical air bag' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lung that I've seen before include "Organ providing oxygen to the blood" , "One of a pair of organs in the ribcage" , "Respiratory organ" , "urban area" , "Of sound mind" .)