Respiratory organ (4)
I believe the answer is:
lung
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">respiratory organ</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that Lung is a type of respiratory organ)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lung that I've seen before include "Respiratory organ" , "Air bag?" , "breather" , "Organ used for breathing" , "One of a pair of organs in the ribcage" .)