A girl scratching back sadly (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'a girl scratching back' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'scratching back' means to remove the last letter (scratch can mean to erase).
'lass' with its final letter taken off is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "Regretfully!" , "Unhappily" , "Proper, respectable" , "Sadly, unfortunately" , "Expression of regret" .)