Single-celled organism (6)
I believe the answer is:
amoeba
'single-celled organism' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for amoeba that I've seen before include "Microscopic organism consisting of a single cell changing constantly" , "Simplest animal form" , "Tiny organism consisting of single changing cell" , "O, Mam, be a one-celled animal" , "Microscopic animal found in water" .)