Area final cut short, regrettably (4)
I believe the answer is:
alas
'regrettably' is the definition.
(I've seen this before)
'area final cut short' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'final' becomes 'last' ('last' can be a synonym of 'final').
'cut short' means to remove the last letter.
'last' with its final letter taken off is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "Regrettably, unfortunately" , "woebegone expression" , "Exclamation of grief" , "Regretfully!" , "Cry of woe" .)