Terrible name for 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)
'tsar' is the second definition.
(name of several Russian tsars)
'for' acts as a link.
(Other definitions for ivan that I've seen before include "Vain (anag) - Tsar of Russia" , "'Terrible' emperor of Russia" , "Russian boy's name" , "Russian ruler - a Terrible one" , "- - the Terrible (Czar)" .)