Ruler of the past (4)
I believe the answer is:
tsar
'ruler of the past' 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 "Once-powerful Russian" , "Title given to the kings of Bulgaria" , "Rats (anag)" , "Person in charge" , "Alexander and Nicholas held this title in Russia" .)