Unicellular organism (6)
I believe the answer is:
amoeba
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unicellular organism</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for amoeba that I've seen before include "Microscopic organism consisting of a single cell changing constantly" , "Unicellular living thing" , "Tiny organism consisting of single changing cell" , "Microscopic organism of single cell constantly changing shape" , "Protozoan of ever-changing shape" .)