Train a chap to be a driver (8)
I believe the answer is:
coachman
'a driver' is the definition.
(I know that coachman is a type of driver)
'train a chap' is the wordplay.
'train' becomes 'coach' (synonyms).
'a chap' becomes 'man' (chap can mean a man).
'coach'+'man'='COACHMAN'
'to be' is the link.
(Other definitions for coachman that I've seen before include "used to appear on the box" , "Horse-drawn carriage driver" , "Wagoner" .)