One-celled organism (6)
I believe the answer is:
amoeba
'one-celled organism' 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 creature found in water" , "One cell" , "Microscopic organism of single cell constantly changing shape" , "Unicellular living thing" , "Protozoan of ever-changing shape" .)