* Thief is a move: Thief is a move in Pokémon that allows a Pokémon to steal an item held by the opponent.
* Clamperl doesn't hold items: Clamperl, by default, doesn't hold any items in the wild or in battle.
Therefore, using Thief on a Clamperl would have no effect.
What you can do instead:
* Use other moves: There are many other moves you could use against Clamperl, depending on your Pokémon's type and strategy.
* Train your Pokémon: If you want to use Thief effectively, you need to find a Pokémon that can learn it and train it to a level where it can use it effectively.
Let me know if you have any other questions about Pokémon!