Old Pole could have been Belgian ruler (7)

I believe the answer is:

leopold

I'm a little stuck... Click here to teach me more about this clue!

'ruler' is the definition.
(I've seen this before)

'old pole could have been belgian' is the wordplay.
'old pole' anagrams to 'LEOPOLD'.
But, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for leopold that I've seen before include "Victoria's relative" , "Mr Bloom in an old Opel" , "Name of several Belgian monarchs" , "Three kings of Belgium" , "Mozart's father" .)

I've seen this clue in the Sydney Morning Herald.

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