Can you cast a creature with flash during your opponent's combat phase after attackers are declared?

Flash (CR 702.8) allows a permanent to be cast any time you could cast an instant. Instants can be cast whenever a player has priority (CR 116.1), which includes during your opponent's combat phase after attackers are declared — specifically during the 'Declare Attackers Step' after the active player receives priority (CR 508.1).

After the active player declares attackers and triggers resolve, both players receive priority in turn (CR 508.1f). At that point, a player holding a creature with Flash may cast it as though it were an instant, because Flash explicitly removes the sorcery-speed restriction (CR 702.8a).

This is one of the most common tactical uses of Flash: you can cast a creature after seeing which creatures are attacking, letting you make an informed blocking decision. The newly cast creature can then be declared as a blocker in the subsequent Declare Blockers Step (CR 509.1), as long as it has no summoning sickness restriction on blocking — blockers do not suffer from summoning sickness (CR 302.6).

Example: Your opponent attacks with a 3/3. You hold a Briarbridge Tracker (a creature with Flash). After attackers are declared and priority is passed to you, you cast Briarbridge Tracker. It enters the battlefield, and in the Declare Blockers Step you assign it to block the 3/3, trading in combat.