Extraterrestrial body (5)
I believe the answer is:
comet
'extraterrestrial body' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for comet that I've seen before include "Halley's space traveller?" , "Small extraterrestrial body" , "astronomical sight" , "Celestial orbiter" , "Line of rotation" .)