A heavenly body has to arrive on time (5)
I believe the answer is:
comet
'a heavenly body' is the definition.
(comet is a kind of heavenly body)
'arrive on time' is the wordplay.
'arrive' becomes 'come' (coming is a kind of arriving).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'come'+'t'='COMET'
'has to' is the link.
(Other definitions for comet that I've seen before include "Shooting star" , "high-flier" , "Heavenly visitor" , "Tailed space traveller" , "Small extraterrestrial body" .)