Trouble Masking my Key

I've done a google search but i've had no luck so far. I feel like im missing something obvious. What im trying to do is only apply my key to a portion of my frame using a garbage mask. From what I understand I have to duplicate the clip, use a garbage mask on the top clip, and then turn the key off for the top clip. Not sure where im going wrong.

Thanks!

you have apply the mask to the clip with the key, that simple
no duplicating ... avoid that 'source' for future reference ....


And: Wellcome to fcp.co

Using the keyed? Here's the info:
help.apple.com/finalcutpro/mac/10.2/#/ver40b0069e

Apply the keyer effect and the mask effect directly to the clip. No need to duplicate.

Do you only want the keyer to affect part of the frame? (like a green car window) or do you want to crop out parts of the frame beyond the edges of a green screen? In the second case, follow Ben and Karsten's instructions,
If it's the first case, what you are looking for is called a 'hold out mask' and you have two options, (there may be more)

1; Duplicate the clip underneath the timeline without the keyer, then apply a mask to the original clip so that only the parts you want the keyer applied to show, and the original clip shows though from underneath.
2; Duplicate the clip above the timeline without the keyer, then apply a mask to the duplicate so that only the parts you don't want keyed show, and the Keyed clip shows though from below for the rest.

Keep in mind that that this may not lead to satisfactory results, as the keying process can mess with the color of the clip, so if that's what you want, you may be headed for more complications involving more duplicate layers and image masking.