Minor planet (8)
I believe the answer is:
asteroid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">minor planet</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for asteroid that I've seen before include "could be dangerous for astronauts" , "Small rocky object orbiting the sun - store aid (anag)" , "Small body orbiting sun" , "space traveller" , "Small planet orbiting the Sun" .)