Sadly in arrears, a labourer's given up (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'in arrears a labourer's given up' is the wordplay.
'in' indicates a hidden word.
'given up' says the letters should be written in reverse (in a down clue, letters go up).
'sala' is hidden within 'arrears a labourers'.
'sala' backwards is 'ALAS'.
(Other definitions for alas that I've seen before include "Expression of grief or misfortune" , "Regrettably, unfortunately" , "Regretfully!" , "By bad luck" , "Proper, respectable" .)