Here are some possible antonyms depending on the meaning:
If "mantle" refers to a loose outer garment:
* Shirt, dress, robe, coat, jacket
* Bare, naked, unclothed
If "mantle" refers to a covering or layer, like the Earth's mantle:
* Surface, crust, exterior, skin, shell
* Core, center, interior
If "mantle" refers to a symbolic responsibility or role:
* Abandon, renounce, forfeit, relinquish, decline
* Ignore, neglect, disregard
If "mantle" refers to a fireplace surround:
* Hearth, fireplace, hearthstone
To give you the best antonym, please provide more context about how "mantle" is being used.