Dwarf planet (5)
I believe the answer is:
pluto
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dwarf planet</span>' is the definition.<br/ ><span class="explanation-format__explanation">(Pluto is a dwarf planet)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for pluto that I've seen before include "It used to be a planet" , "Remote member of solar system" , "Greek god - cartoon dog" , "underworld boss" , "Dog star" .)