Guide one through storyline (5)
I believe the answer is:
pilot
'guide' is the definition.
(I know that pilot is a type of guide)
'one through storyline' is the wordplay.
'one' becomes 'i' (Roman numeral).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'storyline' becomes 'plot' (eg the plot of a film).
'i' placed inside 'plot' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Test or trial (project or programme)" , "One-off programme" , "may be up in the clouds" , "Plane flier" , "Guide - flier" .)