The most common and most likely answer to this crossword clue is the 10 letter word CONNECTIVE
I've seen this crossword clue in The Times
C
O
N
N
E
C
T
I
V
E
Buddy explains!

The straight part
We know that "Joining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joining = connective.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 CONNECTIVE mean?
CONNECTIVE noun- That which connects.
- A function that operates on truth values to give another truth value.
- (grammar) A word used to connect words, clauses and sentences, most commonly applied to conjunctions.
- The tissue which connects the locules of an anthers together.
- A connective tissue.
- Serving or tending to connect; connecting.