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