Shadow boxer, maybe (3)

I believe the answer is:

dog

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'shadow boxer' is the definition.
'dog' can be an answer for 'boxer' (I've seen this in other clues). I am unsure of the 'shadow' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for dog that I've seen before include "St Bernard, for example" , "Domesticated canine" , "Follow - canine" , "Worry" , "Contemptible man" .)

I've seen this clue in The Independent.

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