Vehicle has transformed top jailer (6)
I believe the answer is:
captor
'jailer' is the definition.
(I've seen this before)
'vehicle has transformed top' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'has' means one lot of letters goes inside another.
'transformed' indicates anagramming the letters.
'top' with letters rearranged gives 'pto'.
'car' enclosing 'pto' is 'CAPTOR'.
(Other definitions for captor that I've seen before include "Someone taking prisoner" , "kidnapper, perhaps" , "The one who holds the prisoner" , "Person who has taken another prisoner" , "One who takes a prisoner" .)