Cycle of duty (4)
I believe the answer is:
rota
'cycle of duty' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rota that I've seen before include "Duty roster" , "List assigning turns of duty" , "List of duties to be done, or people to do them" , "Work schedule" , "Register of names for duties" .)