The sixth vehicle for a churchman (5)
I believe the answer is:
vicar
'churchman' is the definition.
(I know that vicar is a type of Priest)
'the sixth vehicle' is the wordplay.
'the sixth' becomes 'VI' (Roman numerals).
'vehicle' becomes 'car' (car is a kind of vehicle).
'vi'+'car'='VICAR'
'for a' acts as a link.
(Other definitions for vicar that I've seen before include "Local priest" , "Parish priest" , "See stand-in" , "Parish clergyman" , "clerical worker" .)