The most common and most likely answer to this crossword clue is the 9 letter word PENDRAGON
I've seen this crossword clue in Financial Times
P
E
N
D
R
A
G
O
N
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The straight part
We know that "Topless 26, legendary king" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case topless 26, legendary king = pendragon.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 PENDRAGON mean?
PENDRAGON noun- Also capitalized as Pendragon: a title assumed by the ancient British chiefs when called to lead other chiefs: chief war leader, chieftain, dictator, despot or king.