Emperor's vehicle circumnavigating westward ocean (6)
I believe the answer is:
caesar
'emperor's' is the definition.
(title used by Roman emperors)
'vehicle circumnavigating westward ocean' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'circumnavigating' means one lot of letters goes inside another.
'westward' is a reversal indicator.
'ocean' becomes 'sea' (synonyms).
'sea' in reverse letter order is 'aes'.
'car' going around 'aes' is 'CAESAR'.
(Other definitions for caesar that I've seen before include "Eg, Augustus, Julius" , "A scare for Roman emperor" , "leader who acts like a dictator" , "US comedian" , "Old Roman leader" .)