Terrible ruler, one with leading position (4)
I believe the answer is:
ivan
'terrible ruler' is the definition.
(tsar of Russia Ivan the Terrible)
'one with leading position' is the wordplay.
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'leading position' becomes 'van' (I've seen this in other clues).
'i'+'van'='IVAN'
(Other definitions for ivan that I've seen before include "Russian, say" , "One Day in the Life of - Denisovich (Solzhenitsyn)" , "Russian ruler - a Terrible one" , "Russian tsar called 'the Terrible'" , "his progress in history was terrible!" .)