Break watch: one’s not always working (4-5)
I believe the answer is:
part-timer
'one's not always working' is the definition.
(someone who works part-time)
'break watch' is the wordplay.
'break' becomes 'part'.
'watch' becomes 'timer' (a watch might be used to time something).
'part'+'timer'='PART-TIMER'
(Other definitions for part-timer that I've seen before include "One working less than a full week" , "One working reduced hours" , "worker on reduced hours" , "Casual worker" , "One not working the whole week" .)