Scheme involves one aviator (5)
I believe the answer is:
pilot
'aviator' is the definition.
(I know that pilot is a type of aviator)
'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 "One-off programme" , "Guide (e.g. of ship through harbour)" , "Small flame that ignites a gas boiler" , "Aviator" , "Experimental scheme; flyer" .)