Extraterrestrial body (5)
I believe the answer is:
comet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">extraterrestrial body</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 comet that I've seen before include "Halley's space traveller?" , "Small extraterrestrial body" , "astronomical sight" , "Celestial orbiter" , "Line of rotation" .)