A rule the Romans had (6)

I believe the answer is:

empire

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'a rule' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for empire that I've seen before include "Eg Holy Roman, British or Ottoman" , "Supreme power over many countries" , "Kingdom" , "In state of New York" , "Widespread political or industrial organisation" .)

I've seen this clue in The Globe and Mail.

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