Story gripping one in very first episode (5)
I believe the answer is:
pilot
'very first episode' is the definition.
(first episode of a TV show)
'story gripping one in' is the wordplay.
'story' becomes 'plot' (plot is a kind of story).
'gripping one' becomes 'i' (Roman numeral. I am not sure about the 'gripping' bit.).
'in' is an insertion indicator.
'plot' going around 'i' is 'PILOT'.
Can you help me to learn more?
(Other definitions for pilot that I've seen before include "Controller of aircraft" , "Navigate" , "Trial broadcast" , "This person can be found in a plane or a boat" , "Flier or helmsman" .)