Emperor’s carriage crossing sea to the north (6)
I believe the answer is:
caesar
'emperor's' is the definition.
(title used by Roman emperors)
'carriage crossing sea to the north' is the wordplay.
'carriage' becomes 'car' (I've seen this in other clues).
'crossing' is an insertion indicator.
'to the north' says the letters should be written in reverse (in a down clue, letters go up or 'northward').
'sea' written backwards gives 'aes'.
'car' going around 'aes' is 'CAESAR'.
(Other definitions for caesar that I've seen before include "See 12" , "See 2" , "See 13 Across" , "Emperor of ancient Rome, like Julius" , "A scare for Roman emperor" .)