Terrible name for a tsar (4)
I believe the answer is:
ivan
I believe this clue is a double definition.
'terrible name' is the first definition.
(ref to Ivan the Terrible)
'for a tsar' is the second definition.
(name of several Russian tsars)
(Other definitions for ivan that I've seen before include "Russian boy's name" , "First tsar of Russia" , "A Russian emperor, quite Terrible" , "Vain (anag) - Tsar of Russia" , "... the Terrible, 16th-century ruler of Russia" .)