Single-celled animal (6)
I believe the answer is:
amoeba
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">single-celled animal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)