Nope
Defiant only activates when the holder's stats are lowered by the opponent, which can be done by moves like Icy Wind or Sticky Web or the ability Intimidate, and as such, moves that lower the user's stats like Close Combat do not activate Defiant.
However, the ability Contrary inverse stat changes, so Close Combat would boost both defenses if the user had Contrary. Contrary Superpower is Malamar's gimmick for example.
Source: Experience
Hope this helps