Russian ruler, once (4)
I believe the answer is:
tsar
'russian ruler once' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tsar that I've seen before include "Despot" , "One associated with 26 [KREMLIN]" , "Absolute ruler" , "Rats to the Russian emperor" , "Tyrant" .)