The most common and most likely answer to this crossword clue is the 4 letter word LARK
I've seen this crossword clue in Financial Times
L
A
R
K
More clues leading to the result LARK
- Mess about in rescue vessel on lake
- Line chest for a bit of fun
- Bird left Noah's vessel
- Bird’s carefree frolic
- Romp with high-flying singer
- . Play in which King Lear goes mad and loses throne ultimately
- Poet not at home to have fun
- Fun time for the early riser?
- Harmless piece of mischief in 17’s vessel on lake
- Frolic in animal sanctuary behind camel’s back
Buddy explains!

The straight part
We know that "Fun time/ the early riser?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fun time/ the early riser? = lark.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 LARK mean?
LARK noun- Any of various small, singing passerine birds of the family Alaudidae.
- Any of various similar-appearing birds, but usually ground-living, such as the meadowlark and titlark.
- (by extension) One who wakes early; one who is up with the larks.
- To catch larks (type of bird).
- A romp, frolic, some fun.
- A prank.
- To sport, engage in harmless pranking.
- To frolic, engage in carefree adventure.