A girl's incomplete expression of regret (4)
I believe the answer is:
alas
'expression of regret' is the definition.
(I've seen this in another clue)
'a girl's incomplete' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'incomplete' means to remove the last letter.
'lass' with its last letter removed is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "Sad expression" , "woebegone expression" , "Exclamation of misfortune" , "Archaic exclamation of sorrow or regret" , "Cry of grief" .)