Tell Me Acting on an anonymous phone call, the police raid a house to arrest a suspected murderer. They don’t know what he looks like but they know his name is John and that he is inside the house. The police bust in on a carpenter, a lorry driver, a mechanic and a fireman playing cards. Without hesitation or communication of any kind, they immediately arrest the fireman. How do they know they’ve got their man?

*I'm not 100% sure*

Because of two things. Don't carpenters, lorry drivers, and mechanics all have name badges? If none of their names are John, that could be a giveaway. And, if it's the middle of the day, a fireman should be at the fire station. Even if he was off, he wouldn't be in fire gear.

your word would be isle.

isle is an island or peninsula especially a small one.

if you add the letter "a" you get aisle, which is a passage between 2 rows of something like chairs, shelves, or something else.