The most common and most likely answer to this crossword clue is the 9 letter word JIU-JITSU
I've seen this crossword clue in Financial Times
J
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Buddy explains!
The straight part
We know that "physical activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case physical activity = jiu-jitsu.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 JIU-JITSU mean?
JIU-JITSU noun- Brazilian jiu-jitsu.
- A method of self-defence established in Japan emphasizing “jū” (soft or gentle) “jutsu” (art or technique). Japanese martial art that utilizes a large variety of techniques in defense against an opponent.
- Brazilian jiu-jitsu, a related art, derived from judo.
- Precision maneuvering.