Guide was first person involved in scheme (5)
I believe the answer is:
pilot
'guide' is the definition.
(pilot can mean a navigator)
'first person involved in scheme' is the wordplay.
'first person' becomes 'I' (first-person pronoun).
'involved in' is an insertion indicator.
'scheme' becomes 'plot' (plot is a kind of scheme).
'i' placed inside 'plot' is 'PILOT'.
'was' is the link.
(Other definitions for pilot that I've seen before include "Plane's operator" , "Test or trial (project or programme)" , "Guide, flyer" , "Flier of aircraft" , "trial episode" .)