The most common and most likely answer to this crossword clue is the 8 letter word OVERLORD
I've seen this crossword clue in The Times
O
V
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R
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D
More clues leading to the result OVERLORD
- Ruler’s operation in wartime
- See way to support past master, above all
- Wartime operation ended by Kitchener, perhaps
- Too much work? Right for a ruler
- Supremo for D-Day operation?
- Ruler‘s sexy lover gets gold and diamonds
- Ruler in saddle having right to oust leader of activists
- Tax excessively? That’s right for a supremo
- Drool dodderingly about clergyman turning up in wartime operation
- Ruler put too much weight on introducing small change
Buddy explains!

The straight part
We know that "Wartime operation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wartime operation = overlord.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 OVERLORD mean?
OVERLORD noun- A ruler of other rulers.
- In the English feudal system, a lord of a manor who had subinfeudated a particular manor, estate or fee, to a tenant.
- Anyone with overarching power or authority in a given domain.
- To rule over; to domineer.