Clip Wings says you have to sacrifice a creature with flying. Assume the opponent has a non-flying creature (Rancid Rats for example) enchanted with Gryff's Boon (creature gains flying). Will Clip Wings kill the enchanted creature (Rancid Rats in this example)?
Yes. A "creature with flying" is any creature that currently has the ability "flying" for any reason - whether its own card text, an enchantment, or even a temporary source like Aerial Maneuver which grants flying until end of turn.