If I play a unit with Ambush in response to a Deathknell trigger during combat cleanup, does it start a new showdown?

Ruling: Yes. When you play a unit via Ambush in response to a Deathknell trigger during combat cleanup, the unit enters the battlefield before the "determine combat result" step. If both players now have units present, the combat ends in "No Result" and a new combat is staged at that battlefield.

Sequence:

• Combat cleanup occurs; your unit dies and its Deathknell trigger goes on the chain; opponent's unit heals
• You play a unit with Ambush in response to the Deathknell trigger
• Chain resolves: Ambush unit enters the battlefield, then Deathknell resolves
• Determine combat result: both players have units present, so "No Result" occurs
• A new combat is staged with the remaining units

Nuances:

• The Ambush unit gains the same designation (Attacker or Defender) as the player who played it, which matches the designation of the unit that died
• Player designations persist throughout the combat phase, so the Ambush unit retains this designation into the newly staged combat