'Terrible' Russian tsar (4)
I believe the answer is:
ivan
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">terrible russian tsar</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)