Nuclear particle (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nuclear 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 proton that I've seen before include "Stable subatomic particle" , "just a little bit" , "Particle in atoms" , "Positively-charged particle" , "Subatomic particle with positive charge" .)