Scheme involves one flyer (5)
I believe the answer is:
pilot
'flyer' is the definition.
(I know that flyer can be written as pilot)
'scheme involves one' is the wordplay.
'scheme' becomes 'plot' (plot is a kind of scheme).
'involves' 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 "Captain of plane or boat" , "who shows the way?" , "This person can be found in a plane or a boat" , "Small flame that ignites a gas boiler" , "Experimental scheme; flyer" .)