Russian ruler until 1917 (4)
I believe the answer is:
tsar
'russian ruler until 1917' 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 "Rats (anag)" , "Potentate" , "Alexander and Nicholas held this title in Russia" , "One associated with 26 [KREMLIN]" , "A former Russian ruler - rats to him" .)