One-celled creature (6)
I believe the answer is:
amoeba
'one-celled creature' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amoeba that I've seen before include "Single-celled form of life of ever-changing shape" , "shape-shifter" , "One-celled animal found in water" , "Microscopic, single-cell creature" , "Single-celled animal" .)