Expression of regret for a girl not finishing (4)
I believe the answer is:
alas
'expression of regret' is the definition.
(I've seen this before)
'a girl not finishing' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'not finishing' means to remove the last letter.
'lass' with its last letter taken off is 'las'.
'a'+'las'='ALAS'
'for' is the link.
(Other definitions for alas that I've seen before include "Expression of grief or misfortune" , "Oh dear!" , "Lackaday" , "express regret thus" , "Proper, respectable" .)