Chest organ (4)
I believe the answer is:
lung
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">chest organ</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lung that I've seen before include "Air bag?" , "Breathing apparatus" , "Organ used for breathing" , "Organ providing oxygen to the blood" , "Respiratory organ" .)