Perhaps male ruler backing Persian king (6)
I believe the answer is:
Here is my best explanation:
'persian king' is the definition.
(I've seen this before)
'perhaps male ruler backing' is the wordplay.
'perhaps male' becomes 'sex'.
'ruler' becomes 'rex' (I've seen this before).
'backing' says the letters should be written in reverse.
'sexrex' reversed gives 'XERXES'.