Period of work (5)
'period of work' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for shift that I've seen before include "Change (gear); dress" , "Alter" , "Switch" , "Move; straight dress" , "Move - working period" .)
(Other definitions for stint that I've seen before include "Allotted period (of work)" , "one's allotted task" , "Prescribed share of work" , "Stretch; be miserly" , "Be niggardly, or fixed amount of work" .)