Skipper's line in the plot (5)
I believe the answer is:
pilot
'skipper's' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'line in the plot' is the wordplay.
'line' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in the' is an insertion indicator.
'i' placed inside 'plot' is 'PILOT'.
Can you help me to learn more?
(Other definitions for pilot that I've seen before include "This person can be found in a plane or a boat" , "Navigator" , "-- Whale; -- light" , "Steer (a ship or plane)" , "Guide - flier" .)