If I cast Jeleva, Nephalia's Scourge and exile a creature with flash, could I cast that creature when she attacks and triggers her ability?
Flash is defined as follows, which makes it sound like the answer is "yes":
702.8a Flash is a static ability that functions in any zone from which you could play the card it’s on. “Flash” means “You may play this card any time you could cast an instant.”
But the following rule for activated abilities makes it uncertain:
602.5e Activated abilities that read “Activate this ability only any time you could cast an instant” mean the player must follow the timing rules for casting an instant spell, though the ability isn’t actually an instant. The player doesn’t actually need to have an instant card that he or she could cast.