Plan to restrict one trial episode (5)
I believe the answer is:
pilot
'trial episode' is the definition.
(episode made to test out a TV show)
'plan to restrict one' is the wordplay.
'plan' becomes 'plot' (plotting is a kind of planning).
'to restrict' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'plot' placed around 'i' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Controller of aircraft" , "One who brings a vessel into port" , "Plane flyer" , "Navigate" , "Aircraft captain" .)