Celestial body will arrive on time (5)
I believe the answer is:
comet
'celestial body' is the definition.
(comet is a kind of celestial body)
'arrive on time' is the wordplay.
'arrive' becomes 'come' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'come'+'t'='COMET'
'will' is the link.
(Other definitions for comet that I've seen before include "Small heavenly body" , "astronomical sight" , "5 [SKYLIGHT], perhaps?" , "Plane" , "Frozen body" .)