Trial episode: one’s gripped by storyline (5)
I believe the answer is:
pilot
'trial episode' is the definition.
(I've seen this before)
'one's gripped by storyline' is the wordplay.
'one' becomes 'I' (Roman numeral).
'gripped by' means one lot of letters goes inside another.
'storyline' becomes 'plot' (I've seen this before).
'i' put into 'plot' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Flier or helmsman" , "experimental programme" , "who shows the way?" , "One who brings a vessel into port" , "This person can be found in a plane or a boat" .)