Space traveller leaves (6)
I believe the answer is:
rocket
I believe this clue is a double definition.
'space traveller' is the first definition.
'leaves' is the second definition.
(I've seen this before)
(Other definitions for rocket that I've seen before include "A slangy reprimand or a firework" , "Go up suddenly" , "Shoot upwards" , "7 or firework" , "go fast" .)