A girl scratching rear, unfortunately (4)
I believe the answer is:
alas
'unfortunately' is the definition.
('alas' is used to express regret)
'a girl scratching rear' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'scratching rear' means to remove the last letter (the letter at the rear of the word is removed).
'lass' with its last letter taken away is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "Cry of woe" , "Sad to say!" , "By bad luck" , "Expression of grief/pity" , "express regret thus" .)