Type of security, carried in vehicle, for ruler (4)
I believe the answer is:
czar
'ruler' is the definition.
(czar is a kind of ruler)
'type of security carried in vehicle' is the wordplay.
'type of security' becomes 'z' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'carried in' indicates putting letters inside.
'vehicle' becomes 'car' (car is a kind of vehicle).
'z' going inside 'car' is 'CZAR'.
'for' is the link.
Can you help me to learn more?
(Other definitions for czar that I've seen before include "Russian ruler (alt. but not rare spelling)" , "Pre-1917 Russian ruler" , "Russian emperor" , "Despot" , "person of great power" .)