One enters a scheme with an airman (5)
I believe the answer is:
pilot
'an airman' is the definition.
(pilot is a kind of airman)
'one enters a scheme' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enters' means one lot of letters goes inside another.
'a scheme' becomes 'plot' (plot is a kind of scheme).
'i' going into 'plot' is 'PILOT'.
'with' acts as a link.
(Other definitions for pilot that I've seen before include "Flier or helmsman" , "-- Whale; -- light" , "Person in charge of aircraft" , "Driver of aircraft" , "Name of Mr Rochester's dog" .)