Can you cast a creature with flash from your hand during combat?

Flash (CR 702.8) grants a permanent the ability to be cast any time you could cast an instant. Normally, creatures can only be cast during your main phase when the stack is empty (CR 307.1), but Flash removes that restriction entirely.

During combat, there are several points where players receive priority and may cast spells: after the beginning of combat step, after declare attackers, after declare blockers, and during the combat damage step (CR 509.1, 510.1, 511.1). At any of these priority windows, a player may cast a creature with Flash.

This is true for both the active player and the non-active player. The non-active player also receives priority at each of these steps (CR 117.3b), so either player can flash in a creature during combat.

Concrete example: Your opponent attacks with a 3/3. You have a Briarbridge Tracker (a creature with Flash) in hand. After attackers are declared and you receive priority, you cast Briarbridge Tracker. It enters the battlefield and you can then declare it as a blocker — but only if this is during the declare blockers step and before blockers have been declared (CR 509.1).

Note: Simply having Flash does not let you declare a creature as a blocker after it enters — you must cast and resolve the Flash creature before the declare blockers step begins in order to be eligible to block (CR 509.1).