The most common and most likely answer to this crossword clue is the 8 plus 3 letter word MUHAMMAD ALI
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More clues leading to the result MUHAMMAD ALI
Buddy explains!
The straight part
We know that "One’s not very good at boxing in silence (something bizarre to behold!)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one’s not very good at boxing in silence (something bizarre to behold!) = muhammad ali.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.