The most common and most likely answer to this crossword clue is the 3 letter word WAG
W
A
G
Buddy explains!

The straight part
We know that "Joker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joker = wag.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 WAG mean?
WAG noun- An oscillating movement.
- A witty person.
- To swing from side to side, such as of an animal's tail, or someone's head, to express disagreement or disbelief.
- To play truant from school.
- To be in action or motion; to move; progress.
- To go; to depart.