Rather fat for boxing? (5)

I believe the answer is:

round

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'rather fat' is the definition.
'round' can be an answer for 'fat' (thesaurus). I am unsure of the 'rather' bit.

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

Can you help me to learn more?

(Other definitions for round that I've seen before include "balls are like this" , "Item of music - single item of ammunition" , "Spherical; song" , "regular series" , "Like the World" .)

I've seen this clue in the Evening Standard.

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