Sadly, a girl loses her last son (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'a girl loses her last son' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'girl' could be 'las' (I've seen this in other clues) and 'las' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for alas that I've seen before include "Expression of regret" , "Sad to say!" , "Unhappily" , "What a shame!" , "Expression of grief/pity" .)