One leaving country before fighter's let in (6)
I believe the answer is:
emigre
'one leaving country' is the definition.
(someone emigrating)
'before fighter's let in' is the wordplay.
'before' becomes 'ere' (poetic term for 'before').
'fighter' becomes 'mig' (fighter jet produced by Russian company MiG).
'let in' means one lot of letters goes inside another.
'ere' going around 'mig' is 'EMIGRE'.
(Other definitions for emigre that I've seen before include "Regime (anag)" , "One living abroad for political reasons" , "outcast?" , "One living abroad (to escape regime)" , "One who has left his country for political reasons" .)