The most common and most likely answer to this crossword clue is the 12 letter word ANTIPATHETIC
I've seen this crossword clue in Independent
A
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More clues leading to the result ANTIPATHETIC
Buddy explains!

The straight part
We know that "23" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 23 = antipathetic.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.
What does ANTIPATHETIC mean?
ANTIPATHETIC adjective- Having or showing a strong aversion or repugnance
- Opposed in nature or character; antagonistic
- Causing a feeling of antipathy; repugnant