Alack! (4)
I believe the answer is:
alas
'alack' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alas that I've seen before include "Cry of grief" , "By bad luck" , "Lackaday" , "Archaic exclamation of sorrow or regret" , "Cry of regret" .)