Do Deathknell effects resolve before combat cleanup?

No, Deathknell effects do not resolve before combat cleanup.

In fact, Deathknell abilities trigger during the combat cleanup process, but their actual resolution occurs only after the combat cleanup (including healing) is fully completed.

According to the official combat cleanup sequence (as detailed in RiftJudge FAQs #10718 and #10444):

1. Combat Damage Step: Combat damage is dealt to units.
2. Triggering Deathknell: Units with lethal damage trigger their Deathknell abilities. These triggers are added to the chain as Pending Items.
3. Killing Units: Units with lethal damage are moved to the trash.
4. Healing: All surviving units on the battlefield are healed.
5. Resolution: The game returns to the FEPR (Finalize, Execute, Pass, Resolve) process, and the Deathknell triggers that were added to the chain in Step 2 finally resolve.

Because the healing step (Step 4) occurs before the Deathknell effects resolve (Step 5), you cannot use Deathknell damage to "finish off" enemy units that survived combat damage, as they will be healed to full health before the effect executes.