Wandering outer space first while travelling (2,5)
I believe the answer is:
en route
'while travelling' is the definition.
(I've seen this before)
'wandering outer space first' is the wordplay.
'wandering' is an anagram indicator.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'first' is a charade indicator (letters next to each other).
'outer' anagrammed gives 'route'.
'route' after 'en' is 'EN ROUTE'.
(Other definitions for en route that I've seen before include "As part of a journey" , "Along the way" , "'On the way (2,5)'" , "In transit (French)" , "On the way - in France" .)