One-celled animal (6)
I believe the answer is:
amoeba
'one-celled animal' 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 "Microscopic organism of single cell that changes shape constantly" , "Microscopic organism of single cell constantly changing shape" , "Protozoan life-form" , "Single-celled organism" , "Tiny one-celled animal in water" .)