Oh no! A girl took Sunday off! (4)
I believe the answer is:
alas
'oh no' is the definition.
(I've seen this in another clue)
'a girl took sunday off' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'sunday' could be 's' and 's' is found in the answer.
The remaining letters 'la' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for alas that I've seen before include "Archaic exclamation of sorrow or regret" , "express regret thus" , "woebegone expression" , "Woeful expression" , "Unhappily" .)