A sleuth is someone who investigates and solves mysteries.
When people think of detectives or mystery writers, they might use the word "sleuth" to describe them. Think of Sherlock Holmes from the classic novels - he's a famous sleuth with incredible skills for solving puzzles and uncovering clues. In everyday life, you might hear someone say "my husband is a bit of a sleuth when it comes to fixing things around the house," meaning they're good at figuring out what's wrong and how to fix it.
A detective.
To act as a detective; to try to discover who committed a crime, or, more generally, to solve a mystery.
