Unusually remote celestial body (6)
I believe the answer is:
meteor
'celestial body' is the definition.
(meteor is a kind of celestial body)
'unusually remote' is the wordplay.
'unusually' indicates an anagram.
'remote' anagrammed gives 'METEOR'.
(Other definitions for meteor that I've seen before include "Body entering earth's atmosphere" , "Glowing heavenly body" , "it appears as a streak of light" , "Remote type of heavenly body" , "Small piece of matter in the solar system" .)