The most common and most likely answer to this crossword clue is the 5 letter word ABUTS
I've seen this crossword clue in The Daily Telegraph
A
B
U
T
S
More clues leading to the result ABUTS
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 = abuts.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 ABUTS mean?
ABUTS verb- To touch by means of a mutual border, edge or end; to border on; to lie adjacent (to); to be contiguous (said of an area of land)
- To border upon; be next to; abut on; be adjacent to.
- To lean against on one end; to end on, of a part of a building or wall.