Part of, as a plot (4)
'part of as a plot' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for in on that I've seen before include "Privy to" , "Aware of - being part of" , "Participating in - knowing about" , "Wise to" , "Sharing with" .)
(Another definition for inon that I've seen is " Privy to".)