Roman leader's vehicle circumvents sinister-looking marine (6)
I believe the answer is:
Here is my best explanation:
'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'.