First, some insight about each ability.
- Intimidate: When brought into battle, it lowers an opponents Attack stat by 1 stage
- Defiant: When any stat is lowered, its Attack raises by 2 stages
In this situation, Intimidate will lower your Attack by 1 stage. Then Defiant will kick in and boost your Attack by 2 stages. -1 + 2 = +1 which means that the Pokemon with Defiant will have +1 Attack at the start of battle.