Absentee who's on shifts (2-4)
I believe the answer is:
no-show
'absentee' is the definition.
(I've seen this in another clue)
'who's on shifts' is the wordplay.
'shifts' is an anagram indicator.
'whos'+'on'='whoson'
'whoson' anagrammed gives 'NO-SHOW'.
(Other definitions for no-show that I've seen before include "One not keeping an appointment - who's on (anag)" , "One not turning up" , "Person with reservation not turning up" , "Absentee" , "Person who fails to cancel -- who's on (anag)" .)