Fish organ (4)
I believe the answer is:
gill
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fish 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 gill that I've seen before include "Breathing organ in fish" , "A little drink" , "Small liquid measure" , "ravine" , "Respiratory organ of fish" .)