The most common and most likely answer to this crossword clue is the 1 plus 5 plus 2 plus 1 plus 3 letter word A BROTH OF A BOY
I've seen this crossword clue in Financial Times
A
B
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T
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More clues leading to the result A BROTH OF A BOY
Buddy explains!
The straight part
We know that "Really great chap/Irish cannibal dish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case really great chap/irish cannibal dish = a broth of a boy.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.