Arrive on time to see celestial object (5)
I believe the answer is:
comet
'celestial object' is the definition.
(I've seen this before)
'arrive on time' is the wordplay.
'arrive' becomes 'come' (coming is a kind of arriving).
'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'
'to see' acts as a link.
(Other definitions for comet that I've seen before include "Orbiting extraterrestrial body" , "Celestial object" , "Shooting star" , "It orbits the sun, with bright centre and long tail" , "Celestial body with luminous tail" .)