Old Russian ruler’s vehicle carrying Zambia’s leader (4)
I believe the answer is:
czar
'old russian ruler's' is the definition.
(another spelling of 'tsar')
'vehicle carrying zambia's leader' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'carrying' is an insertion indicator.
'leader' suggests taking the first letters.
The first letter of 'zambia' is 'z'.
'car' enclosing 'z' is 'CZAR'.
(Other definitions for czar that I've seen before include "Pre-1917 Russian ruler" , "Russian ruler (alt. but not rare spelling)" , "Despot" , "person of great power" , "Russian emperor" .)