As a defender, can I use an action spell when my opponent passes priority after moving to my battlefield?

Ruling: Yes, after the attacker passes priority (or resolves their chain), the defender receives focus and can choose to start a chain with an action spell or pass.

Sequence:

• Combat begins when someone moves to your battlefield
• If there are no attack or defend triggers, the attacker gets focus first
• The attacker can start a chain or pass
• After the attacker resolves their chain or passes, the next player in turn order gets focus (the defender in 1v1)
• The defender can then choose to start a chain or pass
• This process continues until all players pass in succession