You can use a movie clip to mask device font text in another movie clip. (You cannot mask device fonts by using a mask layer on the Stage.) For this movie clip mask to function, the user must have Macromedia Flash Player 6 (6.0.40.0) from Adobe or later.
When you use a movie clip to mask device font text, Flash uses the rectangular bounding box of the mask as the masking shape. That is, if you create a nonrectangular movie clip mask for device font text in the Flash authoring environment, the mask that appears in the SWF file takes the shape of the rectangular bounding box of the mask, not the shape of the mask itself.
For more information on using a movie clip as a mask, see Using movie clips as masks in Learning ActionScript 2.0 in Adobe Flash.
For a sample about device font masking, see the Flash Samples web page at www.adobe.com/go/learn_fl_samples. Download and decompress the Samples zip file and navigate to the Masking\DeviceFontMasking folder to access the sample.