Train fellow to become a driver (8)
I believe the answer is:
coachman
'a driver' is the definition.
(coachman is a kind of driver)
'train fellow' is the wordplay.
'train' becomes 'coach' ('coach' can be a synonym of 'train').
'fellow' becomes 'man' (I've seen this before**).
'coach'+'man'='COACHMAN'
'to become' acts as a link.
(Other definitions for coachman that I've seen before include "Horse-drawn carriage driver" , "used to appear on the box" , "Wagoner" .)