Part of the respiratory system (7)
I believe the answer is:
trachea
'part of the respiratory system' is the definition.
('trachea' is part of 'respiratory system')
This is all the clue.
(Other definitions for trachea that I've seen before include "air passage" , "airway" , "Breathing tube" , "Tube taking inhaled air from the larynx" , "get air supply through this" .)