Area final curtailed, sadly (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'area final curtailed' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'final' becomes 'last' (synonyms).
'curtailed' means to remove the last letter.
'last' with its last letter taken off is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "express regret thus" , "Cry of grief" , "I'm sorry to say" , "Interjection of sorrow" , "Exclamation of misfortune" .)