The most common and most likely answer to this crossword clue is the 6 letter word DYNAST
I've seen this crossword clue in The Sunday Telegraph
D
Y
N
A
S
T
More clues leading to the result DYNAST
- Day to depose a not entirely horrible ruler
- Some ‘daddy’, nasty sort of ruler
- Extremely deadly and mostly unpleasant, as Ivan the Terrible was
- Stand by for dancing bishop having come out as family member
- Ruler returning prohibits any dissent? Not entirely
- Ruler shows extremes of devilry, unpleasant no end
Buddy explains!

The straight part
We know that "as Ivan the Terrible was" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as ivan the terrible was = dynast.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DYNAST mean?
DYNAST noun- A ruler or governor, especially a hereditary ruler or someone who founded or is part of a dynasty.