If a creature has an enchantment, like Defang, and I cast Sheltering Word on that creature, would the other enchantment be removed? Or does it need to be cast in response to the enchantment being played?
No, Auras do not target the creature they are attached to once they are in play. Auras only target a creature while they are a spell on the stack. Yes casting Sheltering Word in response to Defang would make your creature untargetable, and therefore Defang would be countered on resolution.