Cryptic crossword clue
You heard chatting about heartless fool that’s not complimentary 12

The most common and most likely answer to this crossword clue is the 12 letter word UNFLATTERING

I've seen this crossword clue in The Daily Telegraph
U
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Buddy explains!

The straight part
We know that "not complimentary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not complimentary = unflattering.
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.

Crossword Answer definitions

UNFLATTERING verb
  1. To show in a bad light; to portray unfavorably.
UNFLATTERING adjective
  1. Not flattering.

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