Rulers of the pack (5)

I believe the answer is:

kings

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'rulers' is the definition.
(king is a kind of ruler)

I don't understand the rest of the clue.

(Other definitions for kings that I've seen before include "feature of 21/23 [NATIVITY SCENE]" , "Cambridge college" , "two books in > a number of colleges" , "Book (or two) of the Bible" , "Henry James, or Edward Lear?" .)

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

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