If an opponent plays Charm to my unit and I react with Discipline, then as the chain is working from Discipline and I draw a Defy, can I use that Defy as a reaction before the Charm resolves?

Yes. You can play the Defy you drew from Discipline before Charm resolves. This is directly covered by FAQ #7449 (and several supporting FAQs like #718, #9292, and #9861).

Here's how the sequence plays out:

1. Opponent plays Charm → Chain: [Charm].
2. You respond with Discipline (Reaction) → Chain: [Charm (bottom), Discipline (top)].
3. Discipline resolves first (top item). You give +2 Might to a unit and draw a card — that card is Defy.
4. Charm is still on the chain. Per Rule 340.4, the controller of the newest remaining item (your opponent, who controls Charm) gains priority first. If they pass, then you gain priority.
5. Since the state is Closed (Charm is still on the chain) and Defy has [Reaction], you can now play it — targeting and countering Charm.

Nuance: Your opponent gets the first chance to act after Discipline resolves (they control Charm, the sole remaining item). If they play another Reaction or activate an ability, that goes on top of the chain above where you'd place Defy. But if they pass, you can freely play Defy to counter Charm before it ever resolves.