The most common and most likely answer to this crossword clue is the 6 letter word LAUGHS
I've seen this crossword clue in The Guardian
L
A
U
G
H
S
More clues leading to the result LAUGHS
Buddy explains!

The straight part
We know that "fun" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fun = laughs.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 LAUGHS mean?
LAUGHS noun- An expression of mirth particular to the human species; the sound heard in laughing; laughter.
- Something that provokes mirth or scorn.
- A fun person.
- To show mirth, satisfaction, or derision, by peculiar movement of the muscles of the face, particularly of the mouth, causing a lighting up of the face and eyes, and usually accompanied by the emission of explosive or chuckling sounds from the chest and throat; to indulge in laughter.
- To be or appear cheerful, pleasant, mirthful, lively, or brilliant; to sparkle; to sport.
- (followed by "at") To make an object of laughter or ridicule; to make fun of; to deride; to mock.
- To affect or influence by means of laughter or ridicule.
- To express by, or utter with, laughter.