The supremo across Russia, initially?! (4)
I believe the answer is:
tsar
'the supremo' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for tsar that I've seen before include "Rats to the Russian emperor" , "Ruler finally executed" , "Rats (anag)" , "Nicholas II, say" , "Peter the Great, for example" .)