Old Russian leader (4)
I believe the answer is:
tsar
'old russian leader' 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)" , "specific authority" , "Once-powerful Russian" , "Formal rulings on Islamic law; was aft (anag.)" , "Alexander and Nicholas held this title in Russia" .)