Single-celled creature (6)
I believe the answer is:
amoeba
'single-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 "Tiny organism consisting of single changing cell" , "Microscopic organism of single cell that changes shape constantly" , "parasitic type" , "Unicellular organism" , "Basic life form" .)