It depends on the clip as to what method you would need to employ. Such as whether there is camera movement, does the light change etc etc. I would normally tackle this in something like After Effects, BUT, if you have a suitable 'patch' clip and the lighting is the same and there is no movement, then I would suggest using the TextTool to create a feathered mask over the 'patch' clip to select exactly what you need and put this on a higher clip in the timeline than the original.
Declan Smith
http://www.madpanic.tv
After Effects CS6/ FCS3 / Canon XLH1 / Canon 7D / Reason / Cubase
"it's either binary or it's not"