Copies cancelled over Persian king, once (6)

I believe the answer is:

xerxes

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'copies cancelled' is the definition.
I can't tell whether this definition defines the answer.

'over persian king once' is the wordplay.
'over' says the letters should be written in reverse.
'persian' becomes 'sex' (I can't explain this - if you can you should believe this answer much more).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'once' becomes 'ex' (prefix meaning former or onetime).
'sex'+'r'+'ex'='sexrex'
'sexrex' back-to-front is 'XERXES'.

Can you help me to learn more?

(Other definitions for xerxes that I've seen before include "One old king" , "King of Persia, 486-465 BC" , "Name of ancient Persian kings" , "Persian king, son of Darius (bridge of boats)" , "Persian 14" .)

I've seen this clue in The Independent.

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