King of the canine kind? (3)

I believe the answer is:

rex

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

I can't explain the remainder of the clue.

(Other definitions for rex that I've seen before include "King's title" , "See 6" , "Dog's name; king" , "One in the palace" , "King (Latin); cat breed" .)

I've seen this clue in the Evening Standard.

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