Roman leader's vehicle circumvents sinister-looking marine (6)
I believe the answer is:
caesar
'roman leader's' is the definition.
(I've seen this before)
'vehicle circumvents sinister-looking marine' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'circumvents' indicates putting letters inside.
'sinister-looking' is a reversal indicator (text right-to-left - sinister is left in Latin).
'marine' becomes 'sea' (both can mean 'of the sea').
'sea' written backwards gives 'aes'.
'car' placed around 'aes' is 'CAESAR'.
(Other definitions for caesar that I've seen before include "extraction operation" , "Old Roman emperor like Julius" , "Julius, stabbed by Brutus" , "See 2" , "He was killed during the Ides of March" .)