Eighth planet from the sun (7)
I believe the answer is:
neptune
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">eighth planet from the sun</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for neptune that I've seen before include "Third largest of the planets, and sea god" , "Planet, identified 1846" , "revolutionary body" , "Holst piece" , "Roman sea-god" .)