Pre-1917 ruler (4)
I believe the answer is:
tsar
'pre-1917 ruler' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tsar that I've seen before include "Nicholas II was the last" , "Russian leader" , "Supremo" , "Peter the Great, for example" , "Person in charge" .)