Too bad! (4)
I believe the answer is:
alas
'too bad' 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 "Exclamation of misfortune" , "By bad luck" , "Expression of grief/pity" , "Regrettably, unfortunately" , "Unhappily" .)