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' is the 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)" .)