Can you cast a creature spell at instant speed using a flash enabler like Teferi, Mage of Zhalfir?

Normally, creature spells can only be cast during your main phase when the stack is empty and you have priority (a 'sorcery-speed' restriction). This is governed by CR 307.1, which applies sorcery timing to creatures by default.

Flash (CR 702.8a) is a keyword ability that allows a spell or permanent to be cast any time you could cast an instant — i.e., at instant speed. When a card or effect grants flash to your creature spells, those spells bypass the sorcery-speed restriction entirely.

Teferi, Mage of Zhalfir has a static ability that gives all creature cards you own in hand flash. As long as Teferi is on the battlefield, every creature spell you cast gains this benefit. Similar effects include Vedalken Orrery and Vivien, Champion of the Wilds.

Concrete example: You control Teferi, Mage of Zhalfir. Your opponent attacks with a 3/3. In response, during their combat phase, you cast Man-o'-War (a creature spell) at instant speed. It enters the battlefield, bounces a creature back to a hand, and its body can block — something impossible without flash.

Note: Teferi himself must already be on the battlefield for his static ability to apply. If Teferi leaves before you cast your creature spell, the flash bonus disappears (CR 611.2b).