A girl said a word of regret (4)
I believe the answer is:
alas
'a word of regret' is the definition.
(I've seen this in another clue)
'a girl said' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'said' indicates a 'sounds like' (homophone) clue.
'lass' is a homophone of 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "Unhappily" , "Expression of grief or misfortune" , "More's the pity" , "Regrettably, unfortunately" , "Lackaday" .)