Positive or negative particle (3)
I believe the answer is:
ion
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">positive or negative particle</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ion that I've seen before include "Subatomic particle" , "Particle (phys.)" , "Electrically-charged particle" , "Electrically charged particle" , "Charged atom/molecule" .)