"That's too bad" (4)
I believe the answer is:
alas
'that's too bad' 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" , "Exclamation of misfortune" , "Expression of regret" , "What a shame!" , "By bad luck" .)