Part of a series (7)
I believe the answer is:
episode
'part of a series' is the definition.
(a TV series is made up of episodes)
This is all the clue.
(Other definitions for episode that I've seen before include "Part of TV series" , "Chapter or event" , "One instalment of a series" , "Part of a broadcast serial, say" , "Incident forming part of a sequence" .)