Firm scheme includes one fellow navigator (7)
I believe the answer is:
copilot
'navigator' is the definition.
(I know that copilot is a type of aircraft pilot)
'firm scheme includes one' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'scheme' becomes 'plot' (plot is a kind of scheme).
'includes' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'plot' enclosing 'i' is 'pilot'.
'co'+'pilot'='COPILOT'
'fellow' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for copilot that I've seen before include "Second-in-command of aeroplane" , "Airliner crew member" .)