Water protozoan (6)
I believe the answer is:
amoeba
'water protozoan' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for amoeba that I've seen before include "little creature" , "Protozoan of ever-changing shape" , "Constantly-changing one-celled creatures" , "Microscopic animal found in water" , "parasitic type" .)