Press the Sauté button on the Instant Pot® and melt butter. Add celery, carrot, and onion and cook until tender, about 5 minutes. Add garlic, pepper, thyme, and oregano and cook until fragrant, about 30 seconds.
Sprinkle flour over vegetables and mix well, then cook 1 minute until no dry flour remains. Slowly add stock, whisking constantly until smooth, then add bay leaf and potatoes. Press the Cancel button.
Close lid and set steam release to Sealing, then press the Manual button and adjust cook time to 5 minutes.
When the timer beeps, let pressure release naturally, about 15 minutes. Open lid and stir in clams and cream. Discard bay leaf and serve immediately with chives for garnish.