Klutz

Full effect

In battle, this Pokémon cannot use its held item, nor will the item have any passive effect on the battle, positive or negative. This Pokémon also cannot use Fling. The Speed cut from the Iron Ball and the effort items (the Macho Brace, Power Weight, Power Bracer, Power Belt, Power Lens, Power Band, and Power Anklet) is unaffected. Items that do not directly affect the battle, such as the Exp. Share, the Amulet Coin, or the Soothe Bell, work as usual. All held items work as usual out of battle. Other moves that use the held item, such as Natural Gift and Switcheroo, work as usual.

In game description

The Pokémon can't use any held items.

Using Klutz in Cobblemon

Abilities are passive and always active, so Klutz costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.

See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.