What happens first, combat cleanup or deathknell? For example, when a Nidalee beats a Scuttlecrab in Showdown?

In a combat showdown, the Deathknell trigger happens during the Combat Cleanup, but the Deathknell ability resolves after the units have healed.

Here is the precise sequence of events during the Combat Cleanup (following Rules 323, 461, and the official RiftJudge FAQs):

1. Combat Damage Step: Combat damage is dealt to units.
2. Triggering Deathknell: As part of the "Handle outstanding board state" step of the cleanup (Rule 323.4), any units with lethal damage that have Deathknell (such as Scuttle Crab) trigger their abilities. These triggers are placed on the chain as "Pending Items."
3. Killing Units: Units with lethal damage are killed and placed in the trash (Rule 323.5).
4. Healing: Surviving units on the battlefield are healed (Rule 461.1.a.1).
5. Resolution: The game returns to the FEPR process, and the Deathknell abilities that were added to the chain in Step 2 resolve.

Summary

While the trigger for the Deathknell occurs during the cleanup, the effect of the Deathknell (e.g., your opponent revealing their hand) does not execute until after the healing step is finished. Therefore, you cannot use a Deathknell effect to "save" a unit or finish off a unit that survived the combat damage, as the healing will have already cleared the damage by the time the Deathknell resolves.