The most common and most likely answer to this crossword clue is the 7 letter word HIJINKS
H
I
J
I
N
K
S
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The straight part
We know that "Larks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case larks = hijinks.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 HIJINKS mean?
HIJINKS noun- Boisterous activity or behaviour, especially in sport; lively fun.
- Tricky or waggish behavior; mischief.
- An old Scottish parlour game in which people played various parts under penalty of a forfeit.