Boxer so shattered? (3-5)

I believe the answer is:

dog-tired

I'm a little stuck... Click here to teach me more about this clue!

'shattered?' is the definition.

'boxer so' is the wordplay.
'boxer' becomes 'dog' (I've seen this in other clues).
'so' becomes 'tired' (I can't explain this - if you can you should give a lot more credence to this answer).
'dog'+'tired'='DOG-TIRED'

Can you help me to learn more?

(Other definitions for dog-tired that I've seen before include "Extremely weary" , "Exhausted" , "Worn out" .)

I've seen this clue in The Guardian.

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