The most common and most likely answer to this crossword clue is the 13 letter word INSUBORDINATE
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result INSUBORDINATE
- Troublesome British trade union is condemned
- Rebellious Britain’s due no changes
- Disregarding instructions, excessive loan’s accepted
- Disobedient aboard sinking boat, men worried about commotion
- Defiant trade union’s bid curtailed undergoing restructure
- Defiant criminal undoes Britain
- Defiant trade union is out around Britain
- Disobedient in playing outside barn
- Resistant antibodies run amok
- Rude bits on unaired broadcast
Buddy explains!

The straight part
We know that "Defiant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defiant = insubordinate.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 INSUBORDINATE mean?
INSUBORDINATE noun- A person who defies authority.
- Rebellious or defiant to authority.
- Contumacious.