"How sad!” (4)
I believe the answer is:
alas
'how sad' 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 "Sad expression" , "'- Poor Yorick' (Hamlet)" , "Interjection of sorrow" , "Lackaday" , "Proper, respectable" .)