Ivan was a terrible one (4)

I believe the answer is:

tsar

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'ivan' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

(Other definitions for tsar that I've seen before include "Male monarch" , "Powerful man" , "Autocrat" , "Peter the Great, for example" , "Eg, Ivan the Terrible" .)

I've seen this clue in the Evening Standard.

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