Small particles (9)
I believe the answer is:
molecules
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">small particles</span>' is the definition.<br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for molecules that I've seen before include "Small particles of matter" , "Simplest structural units of an element" , "Mouse cell (Anag)" .)