Period noted regularly in diary (3)
I believe the answer is:
day
'period' is the definition.
(I know that day is a type of work time)
'noted regularly in diary' is the wordplay.
'noted regularly in' means one should take alternating letters (letters taken at regular intervals).
The alternate letters of 'diary' are 'DAY'.
(Other definitions for day that I've seen before include "24-hour period" , "Doris -, US actress" , "Unit of time" , "Born yesterday" , "Period of time" .)