Terribly remote celestial object (6)
I believe the answer is:
meteor
'celestial object' is the definition.
'terribly remote' is the wordplay.
'terribly' is an anagram indicator.
'remote' anagrammed gives 'METEOR'.
(Other definitions for meteor that I've seen before include "Small extraterrestrial body" , "rock falling to earth" , "Solar System body" , "fighter" , "Space traveller" .)