The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 letter word FACE TO FACE
I've seen this crossword clue in The Guardian
F
A
C
E
T
O
F
A
C
E
Buddy explains!
The straight part
We know that "Like a couple of missionaries" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like a couple of missionaries = face to face.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 FACE TO FACE mean?
FACE TO FACE noun- A meeting, especially a meeting between two people conducted in person as opposed to a meeting conducted at a distance through technology.
- In one another's presence.
- While physically present.