The most common and most likely answer to this crossword clue is the 9 letter word TRUCULENT
I've seen this crossword clue in Independent
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More clues leading to the result TRUCULENT
- Defiantly aggressive criminal culture Unite oddly dismissed
- Obstreperous uncle moved to stand for society in credit
- Aggressive articulated lorry you hired out?
- Lorry borrowed from you sounds hostile
- Quarrelsome culture developing, not without love
- Defiant leader of union advanced after brief upset
- Vehicle you loaned reportedly uncooperative
Buddy explains!

The straight part
We know that "Defiantly aggressive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defiantly aggressive = truculent.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 TRUCULENT mean?
TRUCULENT adjective- Cruel or savage.
- Deadly or destructive.
- Defiant or uncompromising.
- Eager or quick to argue, fight or start a conflict.