The most common and most likely answer to this crossword clue is the 5 plus 3 plus 6 letter word ROUGH AND TUMBLE
I've seen this crossword clue in The Guardian
R
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Buddy explains!
The straight part
We know that "free for all" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case free for all = rough and tumble.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 ROUGH AND TUMBLE mean?
ROUGH AND TUMBLE noun- Rough activity; fights
- An environment of rough activity
- A person who characteristically engages in such activity
- Engage in rough-and-tumble activity
- Active, vigorous and rough, with the possibility of harm
- Highly competitive