Kidnapper's instruction to make a turn in vehicle (6)
I believe the answer is:
captor
'kidnapper's' is the definition.
(I've seen this in another clue)
'instruction to make a turn in vehicle' is the wordplay.
'instruction to make a turn' becomes 'PTO' ('please turn over' as written on a page).
'in' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'pto' going into 'car' is 'CAPTOR'.
(Other definitions for captor that I've seen before include "He catches someone" , "One who confines another" , "One imprisoning another" , "One who takes a prisoner" , "He takes someone prisoner" .)