Terrible czar (4)
I believe the answer is:
'terrible czar' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ivan that I've seen before include "man who had a terrible reputation" , "his progress in history was terrible!" , "__ Lendl, Czech-born American tennis star" , "in Russian history" , "'Terrible' emperor of Russia" .)