Short holiday beware is up, finishing early (3)
I believe the answer is:
vac
'short holiday' is the definition.
(abbreviation for vacation; 'short' suggests a short form)
I can't explain the remainder of the clue.
(Other definitions for vac that I've seen before include "short holiday" , "which helps with housework" , "brief holiday" , "Break" , "Holiday; cleaner" .)