Russian dynast (4)
I believe the answer is:
tsar
'russian dynast' 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 "Formal rulings on Islamic law; was aft (anag.)" , "Eg, Ivan the Terrible" , "Nicholas II, say" , "Powerful man" , "Ruler finally executed" .)