Arrive ahead of time to identify celestial body (5)
I believe the answer is:
comet
'celestial body' is the definition.
(comet is a kind of celestial body)
'arrive ahead of time' is the wordplay.
'arrive' becomes 'come' (synonyms).
'ahead of' is a charade indicator (letters next to each other) (some letters go in front of others).
'time' becomes 't' (abbreviation).
'come'+'t'='COMET'
'to identify' acts as a link.
(Other definitions for comet that I've seen before include "Frozen body" , "Shooting star" , "potential space invader" , "Celestial object" , "astronomical sight" .)