The most common and most likely answer to this crossword clue is the 3 letter word AND
I've seen this crossword clue in The Guardian
A
N
D
More clues leading to the result AND
- It’s a plus but often comes between partners
- Joiner entering San Diego
- . Applause from Cockney joiner
- Reverse-engineered molecule that might bind some solutions here
- With an initial dividend
- Bandit held moreover
- Centres of gas planets ultimately aligned in conjunction
- Cockney worker is a joiner
- E
- Joiner featured in a Shakespeare play, though not much at the start
Buddy explains!

The straight part
We know that "Joiner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joiner = and.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 AND mean?
AND noun- In rhythm, the second half of a divided beat.
- As a coordinating conjunction; expressing two elements to be taken together or in addition to each other.
- (heading) Expressing a condition.
- Breath.
- Sea smoke; steam fog.
- To breathe; whisper; devise; imagine.