A girl mentioned with regret (4)
I believe the answer is:
alas
'a girl mentioned with regret' is the definition.
I can't judge whether this definition defines the answer.
'a girl mentioned' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'mentioned' indicates a 'sounds like' (homophone) clue.
'lass' sounds like 'las'.
'a'+'las'='ALAS'
Can you help me to learn more?
(Other definitions for alas that I've seen before include "Sad to say!" , "What a shame!" , "Exclamation of grief" , "Exclamation of misfortune" , "Archaic exclamation of sorrow or regret" .)