Can you activate Hidden Within in response to Deathknell triggering during cleanup, before losing control of the battlefield?

Ruling: Yes, you can activate Hidden Within in response to Deathknell before losing control of the battlefield. When cleanup finishes, Deathknell is placed on the chain as a pending trigger, and you must resolve the chain before proceeding to the step where control is lost.

Sequence:

• During cleanup, triggered abilities like Deathknell become pending but cannot be added to the chain yet
• Once cleanup finishes, pending triggers are finalized onto the chain
• The chain must be fully resolved before moving to the control-loss step
• During chain resolution, you still control the battlefield and can activate abilities like Hidden Within
• Only after the chain resolves does the game proceed to losing control of the battlefield

Nuances:

• While cleanup is ongoing, no spells or abilities can be activated - triggers only wait as pending
• Deathknell effectively stops the loss of control until it resolves because the chain must be resolved first