Vehicle carrying unknown leader once (4)
I believe the answer is:
czar
'leader once' is the definition.
I can't judge whether this definition defines the answer.
'vehicle carrying unknown' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'carrying' means one lot of letters goes inside another.
'unknown' becomes 'z' (algebra).
'car' going around 'z' is 'CZAR'.
Can you help me to learn more?
(Other definitions for czar that I've seen before include "Despot" , "Russian ruler (alt. but not rare spelling)" , "policy coordinator" , "person of great power" , "Pre-1917 Russian ruler" .)