Part of a serial (7)
I believe the answer is:
episode
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">part of a serial</span>' is the definition.<br/ ><span class="explanation-format__explanation">('episode' is part of 'serial')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)