Part of a serial (7)
I believe the answer is:
episode
'part of a serial' is the definition.
('episode' is part of 'serial')
This is all the clue.
(Other definitions for episode that I've seen before include "Instalment; event" , "Distinctive incident" , "Incident or event as part of sequence" , "One instalment of a series" , "from longer work" .)