Dog a matchless sort of boxer (6)

I believe the answer is:

shadow

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

I cannot understand how the remainder of the clue works.

(Other definitions for shadow that I've seen before include "Follow - dark patch" , "Reflected image" , "Sort of boxer - dog" , "Dark area from which light is blocked" , "Partial or complete darkness" .)

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

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