Happen to regret accompanying space traveller (4,4)
I believe the answer is:
come true
'happen' is the definition.
(I've seen this in another clue)
'regret accompanying space traveller' is the wordplay.
'regret' becomes 'rue' (synonyms).
'accompanying' means one lot of letters go next to another.
'space traveller' becomes 'comet' (object travelling through space).
'rue' put after 'comet' is 'COME TRUE'.
'to' acts as a link.
(Other definitions for come true that I've seen before include "Happen in reality, as dreams may (4, 4)" , "it's been realised" , "Materialise" , "More cute (anag)" , "Happen in the way that was hoped or forecast" .)