Scheme involves one flier (5)
I believe the answer is:
pilot
'flier' is the definition.
(I know that pilot is a type of flier)
'scheme involves one' is the wordplay.
'scheme' becomes 'plot' (plot is a kind of scheme).
'involves' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'plot' enclosing 'i' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Navigate" , "Plane's operator" , "This person can be found in a plane or a boat" , "Guide (e.g. of ship through harbour)" , "One who brings a vessel into port" .)