Fundamental particle (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fundamental particle</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for atom that I've seen before include "A small but mighty particle" , "Tiny bit" , "A really tiny piece of something" , "Piece of matter in moat" , "The tiniest particle" .)