Well, if it's in your zone, relatively small, and unattended, I'd say that falls under the Mundane Effects category. I.E. if there's nothing that would've prevented you from walking over and grabbing it in the same time frame, it's not a serious spell, and doesn't even cost stress; at worst, this sort of thing is a supplemental action.
If it's bigger, or farther away, or there's something else complicating things (like trying to grab the keys to the cell you're stuck in), it becomes a maneuver - as posted by Bernd.
If somebody is holding the thing, it becomes a two-part operation: a maneuver to disarm, opposed by... probably might, but maybe alertness (if it's something they might not notice moving, like a key hanging off their belt), or potentially weapons or guns for appropriate items. Maybe even athletics, if they describe a defensive action like turning around to impose their body mass between you and the suddenly mobile item.
And then a second action to actually grab the item yourself; if it's relatively small, that's probably a supplemental action, doable as part of the same exchange at the normal cost of -1 to the initial maneuver; if it's large, bulky, critically important for you to have (like a weapon of any stripe), or farther away, I'd require a second full action to actually get the item to you - though if you put a duration on your initial spell, I wouldn't charge extra mental stress for doing so. Though, if you've got a fate point to spare and an appropriate aspect to tag for effect...
This is likely to change with GM, though; some will be more (or less) lenient. It'll also vary by game type; if you're running a more cinematic game, where people casually use guns to disarm folks, drive off bridges and somehow land right-side-up and keep going, etc - then directly grabbing an enemy's weapon off a maneuver is totally appropriate. (And they'll happily do the same to you, given the opportunity.)