how to paint a portion of the frame/clip in fcpx ?

have you tried this free plugin?

http://f1.creativecow.net/2819/fcp-x-effect-bg-pixel-blaster

Rotoscoping could be done in various apps, such as Photoshop.

A more 'modern' way is, to motion-track the unwanted area and with the tracking info apply sort-of 'overlay' .... = Motion5

the quick and dirty way I would do it would be to take a screen shot of the last good frame and export that and/or save it and then lay it on top of the timeline for the next set of bad frames. Use a garbage matte to elliminate everything but the small area you need and soften the edges. If you are sitting still it might be straight forward. There are no drawing tools in FCPx that I know about., You could also open the saved frame in photoshop and matte out everything but that small area and soften it in photoshop and re-import that.

I have an FCPX generator called "Follow The Dot" (it's free) which was designed as an "attention tracker" but can be used to cover a small flaw like you describe. Position, Color, Opacity, Feather and Scale can be keyframed.

download from http://sight-creations.com/fxexchange/FollowTheDot.zip

Assuming you're unfamiliar with keyframing an effect:

Place the generator on your storyline over the effected frames (you can drag out the generator for as long as it needs to appear or size it down shorter if you like.) Move the playhead to the first frame of the generator (use the up/down arrows to advance the playhead to position.) Set the necessary keyframes you will be using (as you pass the mouse over parameters, you'll see a small "diamond" shape appear to the right side -- clicking on that icon will "light it up" yellow wth a small X inside [used to toggle the keyframe back off if necessary.]) Usually, to begin with, that will just be the position (Center) parameter.

You can type the right arrow key to advance the playhead frame by frame or Shift - right arrow to advance 10 frames at a time (usually this is sufficient for a first pass.) Any change in position you make to the "Dot" with the onscreen control (OSC) will automatically have the position keyframed by FCPX (so you don't have to continually go back and set individual keyframes after the first one is set.) Track the spot you're trying to remove with the OSC. If you have to go back and make adjustments, just go to the frames the Dot is "off" and adjust the position. It's all rather fast work once you get used to the "flow".

You can keyframe the color as well, so if you need to match "shadowing", you can have the color change to match the tones in the shirt (and some feathering will help disguise the "effect".) To match colors in your video, left click on the color swatch. The Apple system color picker will launch. Use (click on) the "Magnifier" at the top left corner to pick the color closest to the spot that is the color you need to cover the spot.

You can also scale the dot horizontally and vertically (turn down the disclosure triangle) and each of those directions are independently keyframeable.

If you have any questions, message back.

HTH