'Terrible' Russian tsar (4)
I believe the answer is:
ivan
'terrible russian tsar' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ivan that I've seen before include "'Terrible' emperor of Russia" , "... the Terrible, 16th-century ruler of Russia" , "A Terrible old Russian ruler" , "his progress in history was terrible!" , "A Russian emperor, quite Terrible" .)