Aquatic organism (4)
I believe the answer is:
alga
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">aquatic organism</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 alga that I've seen before include "Primitive non-flowering, mainly aquatic, plant" , "Water plant with no stem or leaves" , "Gala (anag.)" , "Gala class of seaweed" , "Autotrophic organism" .)