Arrive on time to see traveller in space (5)
I believe the answer is:
comet
'traveller in space' is the definition.
(comets travel through space)
'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'
'to see' is the link.
(Other definitions for comet that I've seen before include "traveller covering huge mileage" , "Line of rotation" , "Traveller round sun" , "Halley's phenomenon in space" , "Celestial body with luminous tail" .)