Unfortunately a girl loses one shilling (4)
I believe the answer is:
alas
'unfortunately' is the definition.
('alas' is used to express regret)
'a girl loses one shilling' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'loses' indicates named letters should be taken away.
'one shilling' becomes 's' (abbreviation e.g. Lsd - pounds, shillings, pence).
'lass' with 's' taken away is 'las'.
'a'+'las'='ALAS'
(Other definitions for alas that I've seen before include "Expression of regret" , "Regrettably, unfortunately" , "Oh dear!" , "Too bad, so sad" , "Exclamation of grief" .)