Would it work in a double battle if I have a passimian with defiant and a Eevee or something uses a tail whip to get the attack increase?
Nope, it doesn't work
When a stat of a Pokémon with this Ability is lowered by an opponent, its Attack is increased by two stages. Defiant will not activate if the Pokémon with this Ability lowers its own stats (such as with Close Combat) or if its stats are lowered by an ally
Test / Source
