Vehicle carrying unknown ruler (4)
I believe the answer is:
czar
'ruler' is the definition.
(czar is a kind of ruler)
'vehicle carrying unknown' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'carrying' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'car' placed around 'z' is 'CZAR'.
(Other definitions for czar that I've seen before include "Despot" , "policy coordinator" , "Pre-1917 Russian ruler" , "Russian ruler (alt. but not rare spelling)" , "Russian emperor" .)