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' (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'
'to see' acts as a 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" .)