Crossword Genius

Neptune has been leaking? Hope your trip is safe! (8)

Ross

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" .)

I've seen this clue in The Guardian.
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