One enters scheme to become navigator (5)
I believe the answer is:
pilot
'navigator' is the definition.
(I know that pilot is a type of navigator)
'one enters scheme' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enters' means one lot of letters goes inside another.
'scheme' becomes 'plot' (plot is a kind of scheme).
'i' placed inside 'plot' is 'PILOT'.
'to become' acts as a link.
(Other definitions for pilot that I've seen before include "Navigate" , "Captain of plane or boat" , "One-off programme" , "Airman" , "Steer, test (scheme)" .)