The most common and most likely answer to this crossword clue is the 7 letter word COUPLED
I've seen this crossword clue in The Daily Telegraph
C
O
U
P
L
E
D
Buddy explains!

The straight part
We know that "Joined" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joined = coupled.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 COUPLED mean?
COUPLED verb- To join (two things) together, or (one thing) to (another).
- To join in wedlock; to marry.
- To join in sexual intercourse; to copulate.