Neptune has been leaking? Hope your trip is safe! (8)
I believe the answer is:
godspeed
'safe' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
I don't understand the rest of the clue.
(Other definitions for godspeed that I've seen before include "Expression of good wishes to one starting a journey" , "parting statement" , "Wish for successful journey" , "good luck" , "Old term of farewell" .)