What's the matter? (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">what's the matter?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(atom is a kind of matter)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for atom that I've seen before include "It was once thought to be smallest particle of matter" , "Piece of matter in moat" , "Nucleus surrounded" , "A tiny powerful particle" , "Particle of matter now divided" .)