Can an attacker place an character with Ambush into combat after defenders have been announced?

Ambush only puts characters into play. It doesn't make them participate in challenges, you have to declare them as attackers or defenders normally, or use other card effects to make them participate (such as Ours Is the Fury or Wardens of the North). So, unless you are using such card effects, the latest you can use Ambush to get an extra character in a challenge is before initiating it (as the attacker) or after attackers are declared and before defenders are declared (as the defender).

Thanks for the clarification.

You're welcome.

so after attackers are declared you ambush to go into defence?

so after attackers are declared you ambush to go into defence?

Not quite. After attackers are declared, there is an action window. In that action window, you may Ambush a character into play. Once that action window is closed, you may choose to defend with your newly-Ambushed character (assuming icon eligibility).

so after attackers are declared you ambush to go into defence?

Not quite. After attackers are declared, there is an action window. In that action window, you may Ambush a character into play. Once that action window is closed, you may choose to defend with your newly-Ambushed character (assuming icon eligibility).

Assuming eligibility period (when the time comes for declaring defenders). The freshly-ambushed character could be knelt by another effect during the action window or otherwise rendered ineligible to defend (by an ambushed Fishing Net, for instance - there's a Greyjoy character that allows ambushing Weapon attachments).

What this all boils down to is that Ambush does not affect participation in a challenge. As Khudzlin says above, Ambush only puts characters into play. It does not make the character an attacker or a defender in any challenge that happens to be going on at the time.

In terms of making the Ambushed character an attacker or defender, it's no different than any other character. It's subject to all the same timing, restrictions, etc. that characters who were marshaled normally are subject to.