Periods forming part of other assignments (4)
I believe the answer is:
eras
'periods' is the definition.
(era is a kind of period)
'part of other assignments' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ERAS' is hidden within 'other assignments'.
'forming' is the link.
(Other definitions for eras that I've seen before include "Epochs" , "At least two times" , "typeface" , "Periods of time" , "Long and distinct periods of history" .)