"So it goes" (4)
I believe the answer is:
'so it goes' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alas that I've seen before include "express regret thus" , "What a shame!" , "Interjection of sorrow" , "Expression of grief/pity" , "Exclamation of grief" .)