Corrupting a unit after it is declared as attacker or defender has no influence to the combat. So I'm allowed to declare Clan Rats as attacker and trigger their action afterwards.

Do I have to play Chillwind before my opponent declares his defenders? Or is declaring an action so that I can respond to it by playing Chillwind?

You have to declare it before the defender declares. According to the updated turn sequence diagram, defender declares defenders THEN players take actions. Hence at that point, it's already a defender even if you corrupt it

Thanks Sining, your explanation coincides with what I thought.