Aquatic organism (4)
I believe the answer is:
alga
'aquatic organism' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)