Will the attack Thief work if it defeats enemy Pokemon?

No, the Attack Thief ability will not work if it defeats an enemy Pokemon.

Here's why:

* Attack Thief's Effect: Attack Thief allows a Pokemon to steal the target's held item only if the move that hits the target is a physical attack.

* Defeating a Pokemon: Defeating an enemy Pokemon means the battle is over for that Pokemon. There is no move to consider, as the battle is concluded.

Therefore, even if a Pokemon with Attack Thief defeats another Pokemon, it will not steal the defeated Pokemon's item.