[Clipper]

If the card is not an Item (or another known card type like Races, etc.), then it is typically discarded after use whether it says Usable once only or not. If it is an Item and it does not have the "Usable once only" text, then you are generally meant to keep it in play.

There are also a few other exceptions of Items you discard without it saying "Usable once only", such as Wand of Dowsing which tells you to discard the card, and Golden Wine of Xuthal from Conan which states it's wasted (at least I assume that's the case, as the card is clearly too powerful to keep it in play every turn).