The most common and most likely answer to this crossword clue is the 6 letter word XERXES
I've seen this crossword clue in Financial Times
X
E
R
X
E
S
More clues leading to the result XERXES
- Old king queen covered in kisses
- One ancient ruler the other king backed
- Perhaps male ruler backing Persian king
- Son of Darius succeeded old king from the East
- King rejecting the other king
- Relations with 12 looking up for old 12
- Persian ruler, former king, turned up trumps, ultimately
- Ruler overthrowing Congress and King
- Gender king switched over — one of the ancient kings
- Ruler in double-cross, partly taken in by crooked seer
Buddy explains!

The straight part
We know that "King" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case king = xerxes.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.