Can a defender react with a Hitten card before the attacker can play an action with focus?

No, a defender cannot react with a hidden card before the attacker plays an action with focus.

According to RiftJudge FAQ #6977, the attacker always gains focus first in a showdown. Because the attacker has focus, they have the priority to start a chain by playing an action or reaction card. The defender cannot interrupt the attacker to play a card until the attacker has either played a card or passed their focus.

While it is true that cards played from a "Hidden" state gain the Reaction keyword (as per Rule 737.6), this keyword only grants the permission to be played during closed states or at any time a reaction can be played. It does not override the fundamental rules of priority and focus that dictate who is allowed to act first.

Sequence of play:

1. Showdown begins: The attacker gains focus and priority.
2. Attacker's turn: The attacker may play an action or reaction card to start a chain.
3. Defender's response: Only after the attacker plays a card (starting a chain) or passes their focus can the defender respond. If the attacker plays an action, the state becomes "Closed," and the defender may then use their reaction-speed cards (including those played from Hidden) to respond to that chain.

If the attacker passes their focus without playing anything, the defender then gains the opportunity to start a chain. You cannot "interrupt" the player who currently holds focus to play a reaction unless they have already passed that focus to you.

Note: I could not fully verify this ruling against the retrieved rules.