The most common and most likely answer to this crossword clue is the 8 letter word OLIGARCH
I've seen this crossword clue in The Guardian
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More clues leading to the result OLIGARCH
- Ruler at intervals moulding part of foot
- Sort of ruler in gaol, rich, corrupt
- Foul king oddly displaced by cunning ruler
- Corrupt, rich Olga?
- Ruler over Laks is initially good, then roguish
- Log chair carved for Russian billionaire?
- Regularly visited low-lying, shrewd member of ruling minority
- Crude oil given to government by principal member of ruling elite
- A girl melts, captivated by Salmond’s expression – he’s so rich and powerful
- Elite member’s old cigar crushed in left hand
Buddy explains!

The straight part
We know that "Russian billionaire" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case russian billionaire = oligarch.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 OLIGARCH mean?
OLIGARCH noun- A member of an oligarchy; someone who is part of a small group that runs a country.
- (especially Russia, USA, Europe, or China) A very rich person, particularly with political power; a plutocrat.
- (cosmogony) A protoplanet formed during oligarchic accretion.