The most common and most likely answer to this crossword clue is the 6 letter word CARPUS
I've seen this crossword clue in The Guardian
C
A
R
P
U
S
Buddy explains!

The straight part
We know that "Joint" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joint = carpus.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 CARPUS mean?
CARPUS noun- The group of bones that make up the wrist.