Emperor's vehicle letting in rising water (6)
I believe the answer is:
caesar
'emperor's' is the definition.
(title used by Roman emperors)
'vehicle letting in rising water' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'letting in' means one lot of letters goes inside another.
'rising' says the letters should be written backwards (in a down clue, letters go up).
'water' becomes 'sea' (the sea is water).
'sea' in reverse letter order is 'aes'.
'car' enclosing 'aes' is 'CAESAR'.
(Other definitions for caesar that I've seen before include "Roman ruler" , "leader who acts like a dictator" , "A scare for Roman emperor" , "extraction operation" , "Julius, stabbed by Brutus" .)