Single-cell organism (6)
I believe the answer is:
amoeba
'single-cell 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 "Single-celled form of life of ever-changing shape" , "Freshwater or marine protozoa" , "One-celled animal found in water" , "Single-celled animal" , "little creature" .)