Cryptic crossword clue
Writer, with-it, promotes musical that is definitive 13

The most common and most likely answer to this crossword clue is the 13 letter word AUTHORITATIVE

A
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Buddy explains!

The straight part
We know that "definitive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case definitive = authoritative.
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

AUTHORITATIVE adjective
  1. Arising or originating from a figure of authority
  2. Highly accurate or definitive; treated or worthy of treatment as a scholarly authority
  3. Having a commanding style.

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