Traveller in Russian plane to hold forth (7)
I believe the answer is:
migrant
'traveller' is the definition.
(I know that migrant is a type of traveller)
'russian plane to hold forth' is the wordplay.
'russian plane' becomes 'MiG' (MiG is a Russian aircraft manufacturer).
'to hold forth' becomes 'rant' (I've seen this in other clues).
'mig'+'rant'='MIGRANT'
'in' acts as a link.
(Other definitions for migrant that I've seen before include "Animal moving seasonally" , "One moving to get work" , "relocating" , "Moving across territories" , "Person who has moved residence to another place" .)