Fiend Binder's triggered ability
You would get to tap the only creature defending player controls before they can block.  When you actually declare attacks, what happens is that the Fiend Binder becomes tapped and is attacking a specific player.
That player is the defending player, so at that point the ability goes on the stack and you can tap one of their creatures. Defending player is defined before the defender blocks.


  508.4. If an ability of an attacking creature refers to a defending player, or a spell or ability refers to both an attacking creature and a defending player, then unless otherwise specified, the defending player it's referring to is the player that creature was attacking at the time it became an attacking creature that combat, or the controller of the planeswalker that creature was attacking at the time it became an attacking creature that combat.


In a multiplayer game, even though all opponents are generally considered defending players, the ability can only target a creature that the player this specific creature had attacked controls:


  508.4a. In a multiplayer game, any rule, object, or effect that refers to a "defending player" refers to one specific defending player, not to all of the defending players. (...)