Smallest particle of an element (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">smallest particle</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">an element</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">of</span>' acts as a link.<br/ ></span>
(Other definitions for atom that I've seen before include "Smallest particle in an element able to take part in a chemical reaction" , "It was once thought to be smallest particle of matter" , "Very small object or amount" , "Part of molecule" , "Basic unit of a chemical element" .)