The most common and most likely answer to this crossword clue is the 3 plus 5 plus 1 plus 5 plus 3 letter word I’M DOING A GREAT JOB
I've seen this crossword clue in The Guardian
I
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™
M
D
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A
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B
Buddy explains!
The straight part
We know that "often expressed for his opponent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case often expressed for his opponent = i’m doing a great job.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.