One instalment in a series (7)
I believe the answer is:
episode
'one instalment in a series' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for episode that I've seen before include "Set of incidents" , "Incident in a story or part of a serial" , "One of a related series of events" , "Musical passage" , "Distinctive incident" .)