Can you use flash to cast a creature spell during the combat damage step?

Flash (CR 702.8a) grants a permanent spell the ability to be cast "any time you could cast an instant." Instants can be cast whenever a player has priority (CR 116.1), including during any step of the combat phase — including the combat damage step (CR 510).

Priority is passed among players during every step and phase, including the combat damage step, after state-based actions are checked and triggered abilities have been put on the stack (CR 116.3). This means as long as you have priority during the combat damage step, you may cast a creature spell that has Flash.

It is worth noting that casting a creature spell without Flash during the combat damage step is not normally legal. Standard creature spells can only be cast during your main phase when the stack is empty and you have priority (CR 307.1). Flash overrides this restriction entirely.

Concrete example: Attackers and blockers have been declared, and combat damage has just been dealt. Your opponent's 3/3 is now dying to your 3/3. Before the game moves to the end of combat step, you cast Briarbridge Patrol (which has Flash in some printings) during the combat damage step. This is perfectly legal — Flash lets you cast it as though it were an instant, right there in the combat damage step.

So the short answer is: yes, Flash creatures can be cast during the combat damage step whenever you hold priority, making them a powerful combat trick option.