Regretful word (4)
I believe the answer is:
alas
'regretful word' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alas that I've seen before include "Regrettably, unfortunately" , "woebegone expression" , "What a shame!" , "I'm sorry to say" , "Exclamation of misfortune" .)