Pond protozoan (6)
I believe the answer is:
amoeba
'pond protozoan' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amoeba that I've seen before include "Microscopic organism of single cell that changes shape constantly" , "Microscopic animal found in water" , "Tiny organism consisting of single changing cell" , "Constantly-changing one-celled creatures" , "Microscopic creature found in water" .)