If I use Singularity on a Glasc Mixologist and Pickpocket, can he use the Death Knell to bring the Pickpocket back?

Yes, you can use the Deathknell ability of Glasc Mixologist to bring Pickpocket back from your trash.

Sequence of Events

1. Singularity is played, targeting Glasc Mixologist (and potentially another unit).
2. Singularity resolves, dealing 6 damage to Glasc Mixologist.
3. Since Glasc Mixologist has 5 Might, it takes lethal damage and is marked for death.
4. Cleanup begins (Rule 322).
5. Glasc Mixologist's Deathknell triggers and is added to the chain as a Pending Item (Rule 734.1.d.2).
6. Glasc Mixologist is killed and moved to the trash (Rule 322.4).
7. The Deathknell trigger finalizes on the chain (Rule 322.12).
8. You resolve the Deathknell effect: "You may play a unit with cost no more than [3] and no more than [A] from your trash, ignoring its cost."
9. You choose Pickpocket (which has a base Energy cost of 3) from your trash and play it to the board.

Important Nuances

• "When you play me" triggers: Because you are playing Pickpocket from your trash via the Deathknell effect, its "When you play me" ability will trigger. Playing a card from the trash is still "playing" the card. Once Pickpocket enters the board, its triggered ability is added to the chain (Rule 376.4.a).
• Cost: The Deathknell effect allows you to play the unit "ignoring its cost," so you do not need to pay the 3 Energy for Pickpocket.
• Legality: Pickpocket meets the requirements of the Deathknell effect (cost no more than 3).

Note: I could not fully verify this ruling against the retrieved rules.