One-celled organism (6)
I believe the answer is:
amoeba
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one-celled 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 creature found in water" , "One cell" , "Microscopic organism of single cell constantly changing shape" , "Unicellular living thing" , "Protozoan of ever-changing shape" .)