The most common and most likely answer to this crossword clue is the 4 plus 4 plus 6 letter word TAKE HOLY ORDERS
I've seen this crossword clue in The Guardian
T
A
K
E
H
O
L
Y
O
R
D
E
R
S
Buddy explains!
The straight part
We know that "join them" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case join them = take holy orders.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.