A girl heard a cry of woe (4)
I believe the answer is:
alas
'a cry of woe' is the definition.
(I've seen this before)
'a girl heard' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'heard' shows a homophone (sound like).
'lass' is a homophone of 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "woebegone expression" , "Expression of regret" , "Sad expression" , "Regretfully!" , "Cry of regret" .)