This space traveller will arrive at the end of August (5)
I believe the answer is:
comet
'this space traveller' is the definition.
(object travelling through space)
'will arrive at the end of august' is the wordplay.
'will' says to put letters next to each other (I've seen this in other clues).
'arrive' becomes 'come' (synonyms).
'at the end of' suggests the final letters.
The last letter of 'august' is 't'.
'come'+'t' is 'COMET'.
(Other definitions for comet that I've seen before include "Line of rotation" , "Heavenly body with a fiery tail" , "Long distance traveller" , "Celestial body - early jet aircraft" , "high-flier" .)