Champ among boxers in ruthless competition? (3-3-3)

I believe the answer is:

dog-eat-dog

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'ruthless competition?' is the definition.
(I've seen this before)

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

(Other definitions for dog-eat-dog that I've seen before include "Ruthless pursuit of one's own interests" , "Ruthless competition - self-interest" , "Savagely ruthless" , "Internecine competition" , "Ruthless competition of canine cannibals" .)

I've seen this clue in The Sun.

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