The most common and most likely answer to this crossword clue is the 5 letter word ANTIC
I've seen this crossword clue in The Guardian
A
N
T
I
C
More clues leading to the result ANTIC
Buddy explains!

The straight part
We know that "Lark" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lark = antic.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 ANTIC mean?
ANTIC noun- A grotesque representation of a figure; a gargoyle.
- A caricature.
- (often in plural) A ludicrous gesture or act; ridiculous behaviour; caper.
- A grotesque performer or clown, buffoon.
- To perform antics, caper.
- To make a fool of, to cause to look ridiculous.
- To perform (an action) as an antic; to mimic ridiculously.
- Playful, funny, absurd
- Grotesque, incongruous.
- Grotesque, bizarre
- Having existed in ancient times, descended from antiquity; used especially in reference to Greece and Rome.
- Belonging to former times, not modern, out of date, old-fashioned.
- Designating a style of type.
- Embossed without gilt.
- A pose, often exaggerated, in anticipation of an action; for example, a brief squat before jumping