Particle of matter (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">particle of matter</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 atom that I've seen before include "A tiny powerful particle" , "Smallest unit of matter" , "A tiny particle of matter with a big bang" , "Something very small perhaps" , "Moat (anag.)" .)