Can you cast a sorcery during your second main phase after attackers have been declared but combat hasn't ended?

Sorceries can only be cast during your own turn, during a main phase, when the stack is empty (CR 307.1). Once you move from your first main phase into the combat phase by declaring your intent to attack (or simply passing priority into the beginning of combat step), you are no longer in a main phase until the second main phase begins.

The combat phase has five sequential steps: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat (CR 507–511). Your second main phase does not begin until the entire combat phase — including the end of combat step — has concluded (CR 505.1).

So if attackers have been declared but combat has not ended, you are still inside the combat phase. You cannot cast sorceries at this point. You must wait for all combat steps to resolve and the second main phase to begin before you can cast sorceries again.

You can cast instants and activate abilities with instant speed during combat, since those are allowed any time you have priority (CR 116.1).

Example: You attack with a 3/3 and your opponent blocks with a 2/2. Before combat damage is dealt, you want to cast Divination (a sorcery). You cannot — you must wait until after the end of combat step resolves and your second main phase begins.