* A playful or mischievous act: This is the most common meaning of "lark" in slang. It can refer to a prank, a joke, or just a bit of fun. For example, you might say "We had a right lark at the pub last night" or "He's always up for a lark."
* A wild or exciting time: This meaning is similar to the first, but it emphasizes the sense of excitement and energy. You might say "Let's go out and have a lark" or "The party was a real lark."
* A person who is playful or mischievous: This is a less common usage, but it's sometimes used to describe someone who is always up for a good time. For example, you might say "He's a real lark, that one."
It's important to note that "lark" is an informal word and is not typically used in formal settings.