The most common and most likely answer to this crossword clue is the 10 letter word INTERLINKS
I've seen this crossword clue in The Guardian
I
N
T
E
R
L
I
N
K
S
Buddy explains!

The straight part
We know that "Joins" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joins = interlinks.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 INTERLINKS mean?
INTERLINKS verb- To link together.
- To link (two or more things) together.