Breathing organ (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">breathing organ</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lung that I've seen before include "Of sound mind" , "Organ providing oxygen to the blood" , "Pulmonary organ" , "urban area" , "Respiratory organ" .)